According to rook's book of dermatology tell me about "Acne" in detail.

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Note on source: Rook's Textbook of Dermatology (Wiley-Blackwell) is not in the library database, but the library holds two closely authoritative equivalents - Dermatology, 2-Volume Set, 5th Edition (Griffiths/Barker/Bleiker - same Rook school/tradition, same publisher lineage) and Fitzpatrick's Dermatology. The detailed account below is drawn directly from both, cross-referenced throughout.

Acne Vulgaris - A Detailed Account

1. Definition and Overview

Acne vulgaris is a multifactorial chronic inflammatory disorder of the pilosebaceous unit. The clinical picture ranges widely - from mild comedonal acne to fulminant systemic disease. Most cases present with a pleomorphic array of lesions: comedones, papules, pustules, and nodules with varying extent and severity.

Although the course may be self-limited in many patients, sequelae can be lifelong - including scar formation and significant psychological impairment, especially in young people.

Dermatology 2-Volume Set 5e, p. 717

2. Historical Note

The term "acne" was first used in the sixth century CE by Aetius Amidenus, physician to the Emperor Justinian. Its etymology is debated - it may derive from the Greek acme (peak) or may itself be the original term. It fell out of use until the 1800s, when it re-entered medical dictionaries. In 1842, Erasmus Wilson formally separated acne simplex (acne vulgaris) from acne rosacea.

Dermatology 2-Volume Set 5e, p. 717

3. Epidemiology

Affects approximately 40-50 million individuals per year in the US alone, with an annual US cost of at least $2.5 billion
Globally, acne accounts for ~0.3% of total and ~15% of the dermatologic disease burden
~85% of young people aged 12-24 years are affected - it is effectively a physiological occurrence in adolescence
The age of puberty onset has been dropping, so preadolescent acne (ages 7-11) has become increasingly common
Acne commonly persists well into adulthood: 35% of women and 20% of men in their 30s report acne; 26% of women and 12% of men still affected in their 40s
Prevalence rates in adults: up to 64% in the 20s and 43% in the 30s
After age 50: 15% of women and 7% of men still have acne
White males have a tendency toward more severe nodulocystic disease
Acne is also more severe in patients with the XYY karyotype
Two indigenous populations - one in Papua New Guinea and one in Paraguay - have been described who do not develop acne, possibly due to the absence of a westernized diet
Fitzpatrick's Dermatology, p. 1421; Dermatology 2-Volume Set 5e

4. Genetic Factors

Family history is reported in 62.9-78% of patients
Those with family history tend to be male, have earlier onset, truncal involvement, and scarring
Twin studies: 81% of acne variation is caused by genetic factors vs. 19% environmental; up to 98% of monozygotic twins both have acne vs. 55% of dizygotic twins
The number, size, and activity of sebaceous glands is inherited
Genetic associations have been found with variants in: TNF-α, TNFR2, IL-1A, CYP17, TLR2, TLR4, and components of the TGF-β pathway and androgen metabolism regulators
Genome-wide association studies (GWAS) have further explored this
Fitzpatrick's Dermatology, pp. 1421-1422; Dermatology 2-Volume Set 5e

5. Pathogenesis

The diagram below illustrates the sequential stages of acne development:

Pathogenesis of Acne - from early comedo to nodule/cyst

There are four key interlocking pathogenic mechanisms:

5.1 Follicular Epidermal Hyperproliferation (Microcomedo Formation)

The earliest event is abnormal keratinization of the infundibulum - increased proliferation and retention of corneocytes (hyperkeratosis + increased corneocyte cohesiveness) in the upper sebaceous follicle
This leads to microcomedo formation - the precursor of all acne lesions
The infrafundibulum of acne patients responds more readily to comedogenic stimuli than in non-acne individuals
Exogenous agents that trigger this include: halogenated cyclic hydrocarbons, tar products, some cosmetics, and EGFR inhibitors (cetuximab, gefitinib), which disturb normal follicular differentiation causing excessive follicular hyperkeratosis

5.2 Sebum Production and Androgens

The presence of well-developed sebaceous follicles on the face, upper chest, and upper back corresponds directly to the distribution of acne
Acne patients have larger sebaceous glands and produce more sebum than individuals without acne
Sebum production >1.5-4.0 mg/10 cm² per 3 hours is defined as seborrhea
Growth of sebaceous glands and increased sebum are induced by androgens, particularly DHT (dihydrotestosterone)
- In men: DHT mainly derived from testosterone
- In women: androstenedione is the main precursor
- Conversion of testosterone to DHT is 30 times higher in acne-affected skin than normal skin
Eunuchs lacking testicular function do not develop acne
However, increased sebum alone is insufficient - patients with Parkinson disease have marked seborrhea but no acne
Sebaceous gland androgen receptors respond to testosterone and DHT; DHT has 5-10× greater affinity for the androgen receptor
DHEAS (adrenal origin) rises at adrenarche (age 7-8) and is associated with initial comedonal acne development in prepubescent children
Estrogens in sufficient systemic amounts decrease sebum production but the dose needed exceeds that required to suppress ovulation; lower-dose OCP containing 20-50 mcg ethinyl estradiol is effective by suppressing ovarian androgen production
Sebaceous glands also express PPARs, retinoid receptors, estrogen/progesterone receptors

5.3 Cutibacterium acnes (formerly Propionibacterium acnes)

Sebum in sebaceous glands is sterile with no free fatty acids, but the microenvironment of the infundibulum favors growth of C. acnes and S. epidermidis
In acne patients, C. acnes is quantitatively increased in sebaceous follicles
Bacterial lipases split di- and triglycerides → producing free fatty acids that:
- Alter the pattern of keratinization within the infundibulum
- Are chemotactic for neutrophils
High-dose vitamin B12 supplementation increases porphyrin production by C. acnes, promoting inflammation

5.4 Inflammation and Immune Response

Innate immunity:

C. acnes activates Toll-like receptor 2 (TLR2) and TLR4 on keratinocytes and monocytes
This triggers production of IL-1, IL-8, IL-12, TNF-α
Rupture of the comedo wall incites a vigorous neutrophilic inflammatory response with release of reactive oxygen species (ROS) and lysosomal enzymes
Antimicrobial peptides histone H4 (direct microbial killing) and cathelin (interacts with β-defensins and psoriasin) are secreted locally
Peripheral blood monocytes differentiate to CD209+ macrophages and CD1b+ dendritic cells in response to C. acnes

Adaptive immunity:

CD4+ T cells detected in early inflammatory lesions
Both Th1 and Th17 responses are prominent in vitro and in vivo
Th17 cells recruit neutrophils - contributing to antibacterial activity but also tissue injury
ROS and lysosomal enzyme levels correlate with disease severity

5.5 Scarring Mechanism

Scar formation does not directly correlate with disease severity - it can occur even in mild-moderate acne
C. acnes induces MMP-1 and MMP-9 (matrix metalloproteinases) → matrix degradation
All-trans retinoic acid (ATRA) downregulates MMPs and augments TIMP-1 → shifts phenotype toward matrix preservation, explaining one mechanism by which retinoids improve scarring
In non-scarring lesions: robust initial CD4+ T-cell response that subsides
In scarring lesions: smaller initial CD4+ T-cell infiltrate but high proportion of skin-homing memory and effector T cells with sustained inflammation during resolution

5.6 Diet and Acne

High glycemic load diets may exacerbate acne
Dairy ingestion (especially skim milk and whey protein) is weakly associated with acne
Both raise IGF-1 → possible increase in androgen activity and sebocyte modulation
Molecular interplay of FoxO1 and mTOR-mediated nutrient signaling are implicated
Two indigenous populations (Papua New Guinea, Paraguay) not exposed to westernized diets do not develop acne

6. Clinical Features

6.1 History Taking

Most patients report gradual onset around puberty. An abrupt onset should prompt investigation for:

Drug-induced acne
Androgen-secreting tumor

Hyperandrogenism should be suspected in a female with acne that is:

Severe, jawline/lower face distribution
Sudden onset
Associated with hirsutism, irregular menses, deepening voice, increased libido, or acanthosis nigricans

Drug-induced acneiform eruptions (typically monomorphous - all at same stage): anabolic steroids, corticosteroids, ACTH, phenytoin, lithium, isoniazid, vitamin B complexes, halogenated compounds, EGFR inhibitors, progestin-only contraceptives, DHEA/testosterone.

Perimenstrual flares are common in acne vulgaris - 56% of adult women report worsening before menses.

6.2 Cutaneous Findings and Lesion Types

Primary sites: face (most common), back, chest, and shoulders.

Lesion	Description
Closed comedo (whitehead)	Dilated follicle with no visible opening; keratin/sebum accumulation
Open comedo (blackhead)	Dilated follicular opening; dark color due to oxidized lipids + melanin (not dirt)
Papule	Small red inflammatory lesion without visible purulent content
Pustule	Visible white/yellow purulent content at the surface
Nodule	Deeper, larger (>5 mm), firm, painful inflammatory lesion
Cyst/abscess	Fluctuant, purulent, deep - may communicate
Scar	Atrophic (ice-pick, rolling, boxcar) or hypertrophic/keloid

Comedones are the hallmark of acne vulgaris and their presence is required for the diagnosis.

7. Acne Variants and Subtypes

7.1 Neonatal Acne

Presents within the first few weeks of life
Related to maternal and neonatal androgens
Typically self-limited

7.2 Infantile Acne

Presents between 1-12 months of age
Due to elevated LH stimulating testicular testosterone in infant males; also from neonatal DHEA/DHEAS
May occasionally require treatment

7.3 Acne Fulminans

Severe, abrupt-onset systemic form with ulcerative hemorrhagic lesions, fever, arthralgia, leukocytosis
Can be precipitated by initiating isotretinoin (avoid by starting with low dose + concomitant oral corticosteroids)
Treatment: isotretinoin + oral corticosteroids; TNF inhibitors, IL-1 antagonists, immunosuppressives (azathioprine, cyclosporine); dapsone if associated erythema nodosum

7.4 Acne Conglobata

Severe nodulocystic form with coalescent purulent nodules - no systemic manifestations
Part of the follicular occlusion tetrad: acne conglobata + dissecting cellulitis of scalp + hidradenitis suppurativa + pilonidal sinus
Associated with PAPA syndrome (pyogenic arthritis, pyoderma gangrenosum, acne conglobata) - autosomal dominant, PSTPIP1 mutations
Also PAPASH and PASH syndromes
Treatment: prednisone + low-dose isotretinoin initially

7.5 Solid Facial Edema (Morbihan Disease)

Unusual complication: woody induration and midline facial/cheek swelling from impaired lymphatic drainage + fibrosis (mast cell-mediated)
Does not resolve spontaneously
Treatment: isotretinoin 0.2-1 mg/kg/day for 4-6 months (often 9-24 months required) ± ketotifen or prednisone

7.6 Occupational Acne / Oil Acne / Tar Acne

Caused by occupational exposure to mineral oils, cutting fluids, or tar products causing comedone formation

8. Grading / Severity Classification

The Global Alliance to Improve Outcomes in Acne proposed a severity grading scheme used as a framework for treatment selection:

Grade	Description
Mild	Predominantly comedonal; few papules/pustules; limited to face
Moderate	Mix of comedones, papules, pustules; may involve face + trunk
Severe	Many papules/pustules, nodules; extensive; ± trunk involvement
Very Severe / Nodulocystic	Nodules, cysts, sinus tracts, scarring; may be systemic

9. Diagnosis and Laboratory Workup

Diagnosis is clinical - based on history and physical examination
Laboratory workup is indicated when hyperandrogenism is suspected:
- Especially in children aged 1-7 (midchildhood acne)
- Serum androgens, LH/FSH ratio, DHEAS, free testosterone
- In females with signs of hyperandrogenism: rule out PCOS, congenital adrenal hyperplasia, adrenal/ovarian tumor
Bacterial culture if antibiotic-resistant acne or gram-negative folliculitis suspected

10. Treatment

10.1 Cleansing

Twice-daily washing with a gentle cleanser (syndet preferred - maintains normal skin pH)
Avoid harsh alkaline soaps that disrupt the cutaneous lipid barrier
Medicated cleansers (benzoyl peroxide, salicylic acid, sulfur) are useful for hard-to-reach areas (back)

10.2 Topical Medications

Retinoids

Mechanism: normalize differentiation and proliferation of follicular epithelium → loosen and unseat microcomedones (comedolytic) + anti-inflammatory
Should be applied to entire acne-prone area once daily; tretinoin applied in the evening (photolabile)
Beneficial effects not evident for 6-8 weeks or longer; initial apparent exacerbation may occur (externalization of deeper lesions)

Agent	Key Points
Tretinoin	Prototype; available as 0.025%, 0.05%, 0.1% cream/gel; inactivated by UV and benzoyl peroxide (apply separately at night); microsphere formulation (Retin-A Micro) reduces irritancy
Adapalene	Synthetic; targets RARγ; photostable; can be used with BPO; 0.1% and 0.3% gels; greater tolerability; 0.1% is OTC-approved for age ≥12
Tazarotene	Potent; 0.1% cream/gel/foam; more effective than tretinoin 0.025% but more irritating; short-contact therapy minimizes irritation; avoid in pregnancy
Trifarotene	Newer; comparable efficacy; improved tolerability
Topical isotretinoin	Not available in US; works via intraepithelial isomerization to tretinoin; does NOT suppress sebum; less irritating, slightly less effective than tretinoin

Benzoyl Peroxide (BPO)

Powerful antimicrobial via free oxygen radical release - reduces C. acnes burden
Mild comedolytic properties
Available 2.5-10% in creams, gels, washes, foams
Key advantage: bacteria cannot develop resistance to BPO → ideal for combination with topical/oral antibiotics
Can bleach clothing and hair; may cause contact irritation

Topical Antibiotics

Clindamycin and erythromycin are the most used; erythromycin now limited due to high resistance (~62% global resistance by 1996)
Always combine with BPO to prevent resistance development
Topical dapsone 5% and 7.5% gel: alternative with anti-inflammatory properties

Salicylic Acid

Keratolytic; milder comedolytic; available OTC in washes and leave-on products

10.3 Systemic (Oral) Medications

Oral Antibiotics

Tetracyclines (doxycycline, minocycline, sarecycline) - first-line systemic antibiotics
Macrolides (azithromycin, erythromycin) - second-line; increasing resistance
Trimethoprim-sulfamethoxazole - third-line for resistant cases
Principles: always combine with topical BPO; limit duration to minimize resistance; not for use as monotherapy; antibiotic resistance in C. acnes is a growing problem

Isotretinoin (Oral)

Gold standard for severe nodulocystic or recalcitrant acne
Mechanism: reduces sebaceous gland size and sebum production, normalizes keratinization, reduces C. acnes colonization, anti-inflammatory
Dose: typically 0.5-1 mg/kg/day; cumulative dose of 120-150 mg/kg often cited for durable remission
Key adverse effects: teratogenicity (requires iPLEDGE program in US), dry lips/skin/eyes, elevated liver enzymes, hypertriglyceridemia, possible mood effects
Risk of acne fulminans flare on initiation: start low dose + concomitant corticosteroids in high-risk patients
ATRA modulates MMP/TIMP balance → matrix preservation (mechanism of scar improvement)

Hormonal Therapy (Women)

Combined oral contraceptives (OCPs) containing 20-50 mcg ethinyl estradiol: suppress ovarian androgen production via ovulation suppression
Spironolactone (anti-androgen): blocks androgen receptor in the sebaceous gland; effective for adult female acne
Cyproterone acetate (available outside US): anti-androgen

10.4 Light and Laser Therapies

Blue light (415 nm): targets porphyrins produced by C. acnes → bacterial killing
Red light (633 nm): anti-inflammatory
PDT (photodynamic therapy) with aminolevulinic acid: targets sebaceous glands
Lasers (pulsed dye, Nd:YAG): anti-inflammatory, sebaceous gland targeting
Also used for post-acne scar treatment: CO2 laser, fractional laser resurfacing

10.5 Treatment Algorithm (Summary - Global Alliance Framework)

Severity	First-Line	Second-Line / Adjuncts
Mild comedonal	Topical retinoid	Salicylic acid
Mild-moderate papulopustular	Topical retinoid + BPO ± topical antibiotic	Oral antibiotic if extensive
Moderate-severe papulopustular	Oral antibiotic + topical retinoid + BPO	Switch antibiotic; hormonal therapy in women
Severe nodular/conglobate	Oral isotretinoin	Oral corticosteroids for fulminans; hormonal therapy
Maintenance	Topical retinoid ± BPO	-

11. Psychosocial Impact

Acne has an undeniable psychosocial burden. Affected individuals have an increased likelihood of:

Self-consciousness and social isolation
Anxiety disorders and depression
Suicidal ideation

Early, effective treatment is therefore not cosmetic but medically and psychologically necessary.

Dermatology 2-Volume Set 5e, p. 717

12. Complications

Complication	Details
Scarring	Atrophic (ice-pick, rolling, boxcar) - most common; hypertrophic/keloid in predisposed individuals (more common in darker skin)
Post-inflammatory hyperpigmentation (PIH)	Especially common in patients with darker skin tones
Solid facial edema (Morbihan disease)	Woody midline facial induration - rare but disfiguring
Sinus tracts	In severe conglobata
Psychological sequelae	Depression, anxiety, suicidal ideation

Sources:

Dermatology, 2-Volume Set, 5th Edition (Griffiths, Barker, Bleiker et al.) - Chapters 35, 36, 126
Fitzpatrick's Dermatology, Editors Volume 1 & 2 - Chapter 78

Type 4 hypersensitivity

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Type IV Hypersensitivity (Delayed-Type Hypersensitivity)

1. Definition and Overview

Type IV hypersensitivity - also called delayed-type hypersensitivity (DTH) or cell-mediated hypersensitivity - is the only hypersensitivity type that is not antibody-mediated. It is mediated entirely by antigen-specific T lymphocytes (both CD4+ and CD8+). Because antigen-specific T cells need time to proliferate and secrete cytokines (unlike preformed antibodies), the reaction develops 24-72 hours after antigen exposure in a previously sensitized individual - hence the term "delayed."

The reaction represents an exaggerated interaction between antigen and the normal cell-mediated immune (CMI) mechanism, resulting in tissue-damaging inflammation.

Roitt's Essential Immunology; Robbins, Cotran & Kumar - Pathologic Basis of Disease

2. Historical Context - The Gell and Coombs Classification

The four hypersensitivity types were classified by Gell and Coombs (1963):

Type	Name	Mediator	Onset
I	Immediate / Anaphylactic	IgE + mast cells	Minutes
II	Cytotoxic / Antibody-dependent	IgG/IgM + complement/ADCC	Hours
III	Immune complex	IgG/IgM immune complexes	6-12 hours
IV	Delayed / Cell-mediated	T cells + cytokines	24-72 hours

3. Phases of Type IV Hypersensitivity

Type IV reactions occur in two phases:

Phase 1 - Sensitization (Induction Phase)

On first exposure to an antigen, APCs (dendritic cells, macrophages, Langerhans cells) process and present antigen peptides on MHC class II to naive CD4+ T cells
The cytokine environment at the time of activation determines T-cell differentiation:
- APCs producing IL-12 → differentiation to Th1 subset
- APCs producing IL-1, IL-6, IL-23 → differentiation to Th17 subset
IL-2 secreted by activated T cells functions as an autocrine growth factor → T-cell proliferation
Differentiated effector cells enter the circulation and persist as long-lived memory T cells (years)

Phase 2 - Elicitation (Effector Phase)

On re-exposure to the same antigen, memory T cells recognize it and mount a rapid, amplified response
This phase develops over 24-72 hours
Antigen-specific effector T cells (scarce and taking time to arrive) secrete chemokines that recruit macrophages and other leukocytes to the site
The result: tissue inflammation, induration, and potential tissue damage
Cellular & Molecular Immunology (Abbas); Robbins Pathology

4. Mechanisms of Tissue Injury

There are three distinct pathways by which Type IV reactions cause tissue damage:

Type IV Hypersensitivity - Three Pathways (Th1, Th2, CTL)

4.1 Th1-Mediated Inflammation (CD4+ - Classic DTH)

The predominant pathway in most type IV reactions:

Th1 cells recognizing antigen secrete IFN-γ as the primary effector cytokine
IFN-γ classically activates macrophages ("M1 macrophages"), which:
- Markedly augment phagocytosis and microbial killing
- Express more MHC class II molecules → further antigen presentation
- Produce TNF-α, proteolytic enzymes, reactive oxidants, and leukotrienes
TNF-α and TNF-β activate endothelial cells, enhance vascular permeability, and damage local tissue
The result: macrophage-dominated chronic inflammation with potential granuloma formation

4.2 Th17-Mediated Inflammation

Th17 cells produce IL-17 family cytokines, plus IL-21, IL-22, and GM-CSF
These cytokines recruit neutrophils (dominant effectors in Th17 responses)
Orchestrate local inflammation through: pro-inflammatory cytokine/chemokine release, proliferation of effector cells, amplification of Th2-mediated inflammation
Important in: psoriasis, inflammatory bowel disease, rheumatoid arthritis, multiple sclerosis, SLE, atopic dermatitis, chronic transplant rejection

4.3 Th2-Mediated Inflammation

Th2 cells produce IL-4, IL-5, and eotaxin
These recruit and activate eosinophils → eosinophil degranulation → release of toxic proteins, proteolytic enzymes, cytokines, chemokines, and leukotrienes → tissue damage
The dominant mechanism in chronic asthma and some allergic diseases

4.4 CD8+ T Cell-Mediated Cytotoxicity (CTL Pathway)

CD8+ cytotoxic T lymphocytes (CTLs) recognize antigen (in context of MHC class I) on target cells
Destroy target cells via direct cytotoxicity: perforin/granzyme pathway and Fas-FasL pathway
CD8+ cells also secrete IFN-γ
Dominant mechanism in: viral infections, drug hypersensitivity reactions (e.g., Stevens-Johnson syndrome/TEN), some autoimmune diseases

5. Timeline of DTH Reaction

Time After Antigen Challenge	Events
0-4 hours	Initial neutrophil accumulation around postcapillary venules
~12 hours	T cells and blood monocytes infiltrate, organized in perivenular distribution; endothelial cells enlarge; plasma macromolecules leak
~18 hours	Fibrinogen escapes → converts to fibrin; tissue begins to swell and become firm (induration detectable)
24-48 hours	Peak reaction - maximal induration; predominantly mononuclear cell infiltrate (lymphocytes + monocyte-macrophage series)
>48-72 hours	Neutrophils migrate out; reaction subsides (in most cases)

The characteristic histology - perivascular mononuclear cuffing with lymphocytes and macrophages - contrasts sharply with the neutrophil-dominated Arthus reaction (Type III).

Cellular & Molecular Immunology; Roitt's Essential Immunology

6. Granuloma Formation

When the DTH response is chronic (antigen persists and cannot be cleared, e.g., mycobacteria), activated macrophages accumulate around the antigen and form a granuloma:

Type IV DTH reactions - Mantoux test, granuloma histology, granuloma diagram, contact dermatitis

(a) Mantoux test with erythema and induration. (b) Histology of tuberculous granuloma showing caseous necrosis (CN), epithelioid cells (E), giant cells (G), mononuclear cells (M). (c) Granuloma diagram with Th1 cells, epithelioid cells, multinucleated giant cells, central caseous necrosis. (d) Contact dermatitis to nickel.)

Granuloma Structure:

Central caseous necrosis (in TB; other granulomas may be non-caseating)
Epithelioid cells - activated, fused macrophages with abundant pale cytoplasm (resemble epithelial cells)
Langhans/multinucleated giant cells - formed by fusion of epithelioid cells; nuclei arranged in horseshoe or peripheral ring pattern
Surrounding rim of Th1 lymphocytes
Outer zone of fibrosis

Granulomas occur in: tuberculosis, leprosy, sarcoidosis, Crohn's disease, fungal infections (histoplasmosis), schistosomiasis, foreign body reactions.

7. Subtypes and Variants

7.1 Classic DTH (Tuberculin-Type)

Prototype: Mantoux/PPD test (tuberculin skin test)
Injection of PPD (purified protein derivative of M. tuberculosis) into a previously sensitized individual
Reaction: erythema + induration appearing after several hours, maximal at 24-48 hours, then subsiding
A positive test indicates previous or active TB exposure (cell-mediated immunity established)
Loss of DTH responses to ubiquitous antigens (e.g., Candida, Trichophyton) = anergy → indicator of deficient T-cell function (as in HIV/AIDS)

7.2 Contact Hypersensitivity (Contact Dermatitis)

Caused by haptens - small chemicals that bind covalently to skin proteins → form complete antigens
Common sensitizers: nickel, chromium, cobalt (metals), poison ivy/oak (urushiol), rubber chemicals, fragrances, formaldehyde, paraphenylenediamine (hair dye)
Langerhans cells in the epidermis are the key APCs - they take up the hapten-protein complex and present it to T cells in regional lymph nodes
Clinical: erythematous, pruritic, vesicular rash at site of contact; may spread beyond contact site
Jones-Mote (cutaneous basophil) hypersensitivity - a variant with a high proportion of basophils infiltrating the skin lesion (seen with soluble proteins + incomplete Freund's adjuvant; shorter-lasting)

7.3 Granulomatous Hypersensitivity

Chronic form of DTH due to persistent, non-degradable antigens or poorly soluble particles
Macrophage activation and fusion → epithelioid cell granulomas
Examples: TB, leprosy (tuberculoid type), sarcoidosis, Crohn's disease, fungal infections

8. Clinical Examples and Associated Diseases

Disease / Condition	Antigen Specificity	Mechanism	Key Features
Tuberculin skin test	PPD (M. tuberculosis)	Th1, DTH	Diagnostic test; induration ≥10 mm = positive
Contact dermatitis	Nickel, poison ivy, rubber, etc.	Th1/CTL, haptens	Pruritic vesicular eczema at contact site
Type 1 Diabetes Mellitus	Pancreatic β-cell antigens (insulin, GAD)	CTL + Th1	β-cell destruction → insulin deficiency
Multiple Sclerosis	Myelin basic protein, other myelin antigens	Th1 + Th17	Demyelination; relapsing-remitting course
Rheumatoid Arthritis	Collagen? Citrullinated proteins?	Th17 + Th1 ± antibodies	Synovial inflammation, joint destruction
Inflammatory Bowel Disease	Commensal enteric antigens? Self antigens?	Th17 ± Th1	Crohn's (transmural, granulomas) or UC
Psoriasis	Skin keratinocyte antigens	Th17, IL-17/IL-22	Plaques with silver scale; Munro microabscesses
Transplant rejection	Donor MHC alloantigens	CTL + Th1	Acute and chronic rejection
Graft-vs-Host Disease	Host tissue antigens	Donor T cells (CTL + Th1)	Skin, GI, liver damage after allogeneic BMT
Drug hypersensitivity (SJS/TEN)	Drug-modified peptides	CTL-dominant	Widespread keratinocyte apoptosis, mucosal involvement
Lepromatous/Tuberculoid leprosy	M. leprae antigens	Th1 (tuberculoid) vs anergy (lepromatous)	Cell-mediated vs humoral response determines type
Hashimoto's thyroiditis	Thyroid peroxidase, thyroglobulin	Th1, CTL	Lymphocytic thyroiditis → hypothyroidism
Celiac disease	Gliadin (wheat)	Th1	Villous atrophy in small intestine
Sarcoidosis	Unknown antigen	Th1, non-caseating granulomas	Multi-organ; elevated ACE

Robbins, Cotran & Kumar - Pathologic Basis of Disease; Goldman-Cecil Medicine; Cellular & Molecular Immunology

9. Key Cytokines in Type IV Hypersensitivity

Cytokine	Source	Key Function
IFN-γ	Th1, NK cells, CD8+	Classical macrophage activation; key DTH mediator
TNF-α, TNF-β (LT-α)	Th1, macrophages	Endothelial activation, vascular permeability, local tissue damage
IL-2	Activated T cells	Autocrine T-cell proliferation
IL-12	DCs, macrophages	Drives Th1 differentiation
IL-17A/F	Th17	Neutrophil recruitment; pro-inflammatory
IL-21, IL-22	Th17	Amplify inflammation; epithelial effects
GM-CSF	Th17, T cells	Myeloid cell activation and expansion
IL-4, IL-5	Th2	Eosinophil recruitment/activation (Th2 variant)
Eotaxin	Th2, tissue cells	Eosinophil chemotaxis
IL-23	DCs, macrophages	Maintains Th17 phenotype
MCP-1 (CCL2)	Various	Monocyte/macrophage recruitment

10. Comparison with Other Hypersensitivity Types

Feature	Type I	Type II	Type III	Type IV
Mediator	IgE	IgG/IgM	Immune complexes	T cells
Onset	Minutes	Hours	6-12 hours	24-72 hours
Complement?	No	Yes	Yes	No
Key cell	Mast cell, basophil	NK cells, phagocytes	Neutrophils	Macrophages, T cells, eosinophils
Transfer	Serum	Serum	Serum	Cells (not serum)
Example	Anaphylaxis	Hemolytic anemia	Serum sickness	Tuberculin test, contact dermatitis

A key experimental distinction: Type IV hypersensitivity can be transferred passively only with T cells (not serum), whereas Types I-III can be transferred with serum.

11. Diagnosis and Testing

Test	What It Detects	Example
Intradermal skin test (Mantoux/PPD)	Prior sensitization to M. tuberculosis	Read at 48-72 hours; induration ≥10 mm positive
Patch test	Contact sensitizers (haptens)	Applied for 48 hours; read at 48-96 hours
Lymphocyte transformation test (LTT)	Drug-specific T-cell proliferation	In vitro test for drug hypersensitivity
Loss of DTH (anergy panel)	T-cell immunodeficiency	Candida, Trichophyton, mumps antigens
Skin biopsy	Histology: mononuclear perivascular infiltrate vs neutrophils	Distinguish from Type III (Arthus)

12. Clinical Significance of Anergy

Loss of DTH responses (anergy) is seen in:

HIV/AIDS - declining CD4+ T-cell count → loss of recall antigen responses
Severe malnutrition
Sarcoidosis (paradoxically - despite systemic granulomas)
Lymphoma
Immunosuppressive therapy

Clinically used as a marker of cell-mediated immune competence.

Sources:

Robbins, Cotran & Kumar - Pathologic Basis of Disease (p. 206)
Cellular and Molecular Immunology (Abbas) - Chapter 19
Roitt's Essential Immunology - Chapter 14
Goldman-Cecil Medicine - Chapter 36

Psoriasis and lichen planus

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Psoriasis and Lichen Planus

PART I — PSORIASIS

1. Definition

Psoriasis is a common, chronic, recalcitrant inflammatory skin disease characterized by discrete plaques with adherent micaceous (silvery) scales occurring at sites of predilection, particularly sites of minor skin trauma. It is a T-cell mediated, immune-driven disease with both genetic and environmental contributors, and is now recognized as a systemic disease in which skin inflammation is the most dramatic manifestation but cardiovascular, metabolic, and articular involvement also occur.

The WHO (2014) recognized psoriasis as a "chronic, noncommunicable, painful, disfiguring and disabling disease for which there is no cure."

Dermatology 2-Volume Set 5e; Fitzpatrick's Dermatology

2. Historical Note

Hippocrates grouped dry scaly eruptions under "lopoi" (probably including psoriasis and leprosy)
The word "psora" was first used by Galen (129-99 BCE) for a scaly skin condition (though this was likely eczema)
Robert Willan (1809) gave the first accurate description of psoriasis
Ferdinand Hebra (1841) definitively separated psoriasis from leprosy
Heinrich Koebner (1879) described development of psoriatic plaques at sites of skin injury - the Koebner phenomenon

3. Epidemiology

Prevalence: ~2% of the world's population; in the US and Canada as high as 4.6-4.7%
Lower prevalence (0.4-0.7%) in Africans, African-Americans, Norwegian Lapps, and Asians
Onset can occur at any age (infancy to eighth decade)
Two peaks of onset: 20-30 years (Type I) and 50-60 years (Type II)
In ~75% of patients, onset is before age 40; in 35-50%, before age 20
Age of onset earlier in women than men; natural history similar - chronic with intermittent remissions
Remissions lasting ≥5 years reported in ~15% of patients
Psoriatic arthritis affects 5-30% of patients with cutaneous psoriasis
73% have mild-moderate disease; 27% severe involvement
79% of patients have nail changes
Dermatology 2-Volume Set 5e, p. 166; Robbins Pathology

4. Genetic Factors

Positive family history reported in 35-90% of patients
Type I psoriasis: onset before age 40, HLA-Cw6 association, more severe, familial
Type II psoriasis: onset after age 40, no strong HLA association, less severe
GWAS have linked psoriasis risk to polymorphisms in HLA loci and genes encoding proteins involved in:
- Adaptive immunity
- TNF signaling
- Skin barrier function
Several loci also associated with psoriatic arthritis
The concordance among identical twins is high, confirming strong genetic predisposition

5. Pathogenesis

Psoriasis is a T cell-mediated inflammatory disease, presumed to be autoimmune in origin (though the initiating antigens are not defined - may be self antigens, environmental antigens, or both).

The IL-23/Th17 Axis - central to psoriasis:

Antigen presentation: Dendritic cells (DCs) in the dermis process and present antigens to T cells
DC activation: Dermal plasmacytoid DCs and myeloid DCs produce TNF-α and IL-23
IL-23 (heterodimer of IL-23-specific p19 + p40 shared with IL-12) drives Th17 cell development
Th17 cells produce IL-17, which promotes:
- Neutrophil-predominant inflammation
- Keratinocyte hyperproliferation
- Antimicrobial peptide production (defensins, S100 proteins)
Th1 cells produce IFN-γ → macrophage activation → further inflammation
Neutrophils are recruited into the epidermis → form Munro microabscesses in the stratum corneum
Chemokines (IL-8/CXCL8, CTACK, TARC, MCP-1, RANTES) regulate the complex recruitment of both neutrophils (forming Munro abscesses) and lymphocytes via attachment to endothelial cells and epidermotropism

Evidence for immune origin:

Cyclosporine (calcineurin inhibitor) is highly effective → proves T-cell centrality
Anti-IL-17 biologics and anti-IL-23 biologics are highly effective
Anti-TNF-α antibodies (infliximab) clear psoriasis
Fitzpatrick's Dermatology; Robbins Pathology, p. 206; Dermatology 2-Volume Set 5e

6. Triggering Factors

Trigger	Mechanism
Koebner phenomenon	Psoriatic plaques develop at sites of local trauma (scratch, surgery, tattoo)
Streptococcal throat infection	Classically triggers guttate psoriasis
Stress	Psychogenic exacerbation
Medications	Beta-blockers, lithium, antimalarials, NSAIDs, IFN-α, abrupt withdrawal of corticosteroids, TNF-inhibitor discontinuation
HIV infection	Paradoxically severe psoriasis
Alcohol and smoking	Exacerbate disease
Sunburn	Isomorphic response (Koebner)

7. Clinical Features

7.1 Primary Lesion

The classic lesion is a well-demarcated, erythematous plaque covered by loosely adherent, silvery-white (micaceous) scales.

Chronic psoriasis - Clinical appearance (light and dark skin) and histology

(A) Erythematous psoriatic plaques with silvery-white scale on lighter skin. (B) In darker skin, plaques range from salmon-colored to hyperpigmented with gray scale. (C) Histology showing psoriasiform hyperplasia, parakeratosis, and Munro microabscesses (arrows).

Key clinical signs:

Auspitz sign: Removal of silvery scales reveals a glistening wet surface with characteristic pinpoint bleeding - this reflects elongated capillary loops in the dermal papillae plus thin suprapapillary epidermis
Koebner phenomenon (isomorphic response): New plaques appear at sites of skin trauma
Woronoff's ring: A pale blanching ring surrounding psoriatic lesions

7.2 Sites of Predilection

Elbows (extensor aspect) - classic
Knees (extensor aspect) - classic
Scalp (posterior)
Lumbosacral area / intergluteal cleft
Genitalia (up to 45% of patients)
Palms and soles; nails; umbilicus

7.3 Subtypes / Clinical Forms

Subtype	Description
Chronic plaque psoriasis (psoriasis vulgaris)	Most common (80-90%); symmetric, sharply defined erythematous scaly plaques; chronic and stable
Guttate psoriasis	Small (0.5-1.5 cm), drop-like papules scattered over trunk; often in children/young adults post-streptococcal infection; acute onset
Inverse (flexural) psoriasis	Smooth, shiny erythematous plaques WITHOUT scale in body folds (axillae, inframammary, groin, intergluteal); scale absent due to moisture
Pustular psoriasis	Sterile pustules on erythematous base; generalized (von Zumbusch) = fever, systemic illness; localized (palmoplantar)
Erythrodermic psoriasis	Generalized erythema and scaling affecting >90% BSA; medical emergency (thermoregulatory failure, hemodynamic instability)
Nail psoriasis	Pitting (most common), onycholysis, oil-drop sign, subungual hyperkeratosis, splinter hemorrhages; in ~79% of patients
Scalp psoriasis	Thick adherent scale, often extending beyond hairline ("crown of thorns")

7.4 Psoriatic Arthritis

Occurs in 5-30% of cutaneous psoriasis patients; in 10-15%, arthritis precedes skin disease.

Five major types (Moll and Wright classification):

Type	Features
Mono/asymmetric oligoarthritis (most common)	DIP and PIP joints; "sausage digit" (dactylitis) from DIP+PIP involvement of one digit
DIP joint arthritis	Classic but uncommon; exclusive DIP involvement
Rheumatoid arthritis-like	Symmetric polyarthritis; distinguish from RA by RF negativity
Arthritis mutilans	Severe, destructive; "pencil-in-cup" deformity; telescoping digits
Spondylitis/sacroiliitis	Axial disease; often asymmetric

Key distinction from RA: Psoriatic arthritis is seronegative (RF usually negative); involves DIPs; associated with nail changes; asymmetric distribution.

8. Histopathology

Three phases - initial, active, and stable:

Initial lesion

Superficial perivascular lymphocyte/histiocyte infiltrate; papillary edema; dilated capillaries
Mild acanthosis without parakeratosis; no neutrophils yet

Active lesion (diagnostic)

Spongiform pustule of Kogoj: Collection of neutrophils within the upper stratum spinosum
Munro microabscess: Neutrophils/remnants in the stratum corneum surrounded by parakeratosis
These two are pathognomonic for psoriasis (and AGEP)
Tortuous, increased capillaries in papillary dermis; edema at papillary tips
Mixed perivascular infiltrate of lymphocytes, histiocytes, neutrophils
Parakeratosis (retained nuclei in stratum corneum); absent granular layer (hypogranulosis)

Stable/Chronic lesion

Psoriasiform hyperplasia: Marked acanthosis with regular downward elongation of rete ridges ("test tubes in a rack" pattern, "squared-off" rete ridges)
Elongated club-shaped dermal papillae with dilated tortuous capillaries extending superficially
Thin suprapapillary plate (explains Auspitz sign)
Parakeratosis + remnants of Munro microabscess
Hypogranulosis (absent granular layer)
Predominantly lymphocytic infiltrate
Dermatology 2-Volume Set 5e, p. 172; Robbins Pathology

9. Severity Assessment

PASI (Psoriasis Area and Severity Index): Combines body surface area involved (in 4 regions: head, upper extremities, trunk, lower extremities) with clinical grading of erythema, induration, and scaling in each region.

Mild: PASI <10
Moderate-Severe: PASI ≥10

10. Treatment

Topical (Mild disease)

Agent	Mechanism / Notes
Corticosteroids	First-line; reduce inflammation; classes I-VII used based on site
Vitamin D analogues (calcipotriol/calcitriol)	Inhibit keratinocyte proliferation; combination with corticosteroids synergistic
Tazarotene (topical retinoid)	Normalizes keratinocyte differentiation; reduces scale
Calcineurin inhibitors (tacrolimus, pimecrolimus)	For sensitive areas (face, flexures); steroid-sparing
Anthralin (dithranol)	Historical; anti-proliferative; stains clothing
Coal tar	Anti-inflammatory, anti-proliferative; Goeckerman regimen
Salicylic acid	Keratolytic; removes scale to enhance penetration of other agents

Phototherapy (Moderate disease)

Narrowband UVB (NB-UVB, 311-313 nm): First-line phototherapy; immunosuppressive
PUVA (psoralen + UVA): More effective than NB-UVB; higher risk of skin cancer
Excimer laser (308 nm): For localized plaques

Systemic Conventional (Moderate-Severe)

Drug	Notes
Methotrexate	Antifolate; anti-inflammatory; weekly dosing; monitor liver (fibrosis risk); contraindicated in pregnancy
Cyclosporine	Calcineurin inhibitor; rapid onset; monitor BP and renal function; limited to short courses
Acitretin	Oral retinoid; especially effective for pustular/erythrodermic psoriasis; highly teratogenic
Apremilast	PDE4 inhibitor; oral; well-tolerated; modest efficacy

Biologic Therapies (Moderate-Severe / refractory)

Drug	Target	Notes
Infliximab, adalimumab, certolizumab, etanercept	TNF-α	Effective; screen for TB before use
Secukinumab, ixekizumab	IL-17A	Highly effective (PASI 90/100 rates); fast onset
Bimekizumab	IL-17A + IL-17F	Newer; very high efficacy
Ustekinumab	IL-12/23 (p40)	Moderate-high efficacy; q12-weekly dosing
Guselkumab, risankizumab, tildrakizumab	IL-23 (p19)	Very high efficacy; selective IL-23 blockade

JAK Inhibitors (Oral targeted therapy)

Deucravacitinib (TYK2 inhibitor): FDA-approved for moderate-severe psoriasis
Brepocitinib (TYK2/JAK1): Under evaluation

PART II — LICHEN PLANUS

1. Definition and Synonym

Lichen planus (LP) (synonym: Lichen ruber planus) is an idiopathic, chronic inflammatory disease of the skin, mucous membranes, hair follicles, and nails. It is the prototype of lichenoid dermatoses - a group of conditions sharing a characteristic lichenoid tissue reaction histologically.

It is a T-cell mediated autoimmune disorder in which CD8+ T cells induce basal keratinocyte apoptosis, with CD4+ Th1 cells driving the inflammatory response.

The term was introduced by Erasmus Wilson in 1869 (previously named "leichen ruber" by Hebra).

Dermatology 2-Volume Set 5e; Andrews' Diseases of the Skin; Robbins Pathology

2. Epidemiology

Affects 0.2-1% of the adult population (cutaneous); oral LP up to 1-4%
Cutaneous LP: 0.3% of men, 0.1% of women; Oral LP: 1.5% of men, 2.3% of women
No overt racial predisposition; HLA associations include HLA-DR/DQ, HLA-A3
Gene associations: MTHFR mutations; TNF-α, NRP2, IGFBP4 polymorphisms
In Europeans: onset primarily after age 20, peak 40-70 years; very rare after age 80
Childhood LP: 5% or less in Europeans; 10-20% in Indian subcontinent, Arab countries, Mexico
In the UK, Indians account for 80% of childhood LP
May be familial in rare cases

3. Etiology and Associations

LP is idiopathic but associated with multiple triggers:

Association	Notes
Hepatitis C virus (HCV)	Strong association; LP may clear with HCV treatment (some conflicting results with newer antivirals)
Hepatitis B vaccination	Associated with LP onset; lesions typically appear ~1 month after 2nd vaccine dose
Medications (lichenoid drug eruptions)	ACE inhibitors, thiazide diuretics, antimalarials, quinine, gold, NSAIDs, beta-blockers; some with photodistribution
Dental materials	Amalgam restorations associated with oral LP
Chronic GVHD	LP-like lesions due to alloreactive T cells
Immune checkpoint inhibitors (ICIs)	Lichenoid eruptions; managed with topical corticosteroids ± brief drug holiday
Thyroid autoimmunity	Personal/family history in up to 30% of vulvar LP; up to 40% have autoantibodies
Dyslipidemia / T2DM	LP associated with elevated triglycerides, LDL, insulin resistance, frank T2DM

4. Pathogenesis

LP is characterized by a Th1 immunologic reaction with CD8+ T-cell predominance at the dermoepidermal junction:

Basal keratinocytes express altered self-antigens (modified by viral or drug antigens) on their surface
CD8+ cytotoxic T cells (CTLs) recognize these antigens → induce keratinocyte apoptosis
CD4+ Th1 cells contribute: increased Th1 cytokine expression (IFN-γ, TNF-α)
Memory T cells recognize antigen peptides on Langerhans cells or dermal dendritic cells → proliferation and cytokine release
Th1 cytokines attract and activate macrophages
Th2 cytokines attract eosinophils (in some variants)
The LP-like lesions in chronic GVHD (where alloreactive T cells recognize foreign MHC) support the autoimmune hypothesis

5. Clinical Features

5.1 The 6 Ps - Classic Description

"Pruritic, Purple (violaceous), Polygonal, Planar (flat-topped) Papules and Plaques"

5.2 Primary Lesion

Small, flat-topped, polygonal violaceous papules - almost pathognomonic
Initial lesions are erythematous; well-developed = violaceous; resolving = hyperpigmented (especially in darker skin)
Surface: glistening and dry with scant adherent scales
Wickham striae: Gray or white puncta or streaks on the surface (more easily seen with dermoscopy); represent focal hypergranulosis
Lesions range from pinpoint papules to 0.5-1.5 cm plaques

Lichen planus - Wickham striae visible dermoscopically

5.3 Sites of Predilection

Flexor wrists (classic)
Medial thighs, shins, dorsal hands
Trunk and presacral area
Genitalia (glans penis - annular lesions; vulva)
Face: rarely involved (confined to eyelids and lips when present)
Palms and soles (small papules or hyperkeratotic plaques)
Lesions are bilateral and relatively symmetric

5.4 Koebner Phenomenon

LP shows the Koebner (isomorphic) phenomenon - new lesions develop at sites of skin injury, similar to psoriasis and vitiligo.

5.5 Pruritus

Pruritus is often prominent and may precede skin lesions
May be disproportionately severe relative to the amount of skin involvement
Patients tend to rub rather than scratch, so scratch marks are usually absent
Linear LP papules can indicate prior scratching and Koebnerization

5.6 Nail Involvement

Occurs in 5-10% of patients; fingernails > toenails; may be the initial manifestation (especially in children).

Longitudinal ridging and splitting (most common - 90% of nail LP)
Onycholysis and subungual debris
Red lunulae (in ~30%)
Complete matrix involvement → obliteration of nail plate (anonychia)
Pterygium unguis - obliteration of the proximal nail fold with forward growth of cuticle fusing with the nail plate; pathognomonic of nail LP

5.7 Oral Mucosa

Involved in ~70% of cases
Manifests as white papules with reticulate (netlike/lacelike) pattern - often called "oral Wickham striae"
Erosive/atrophic and bullous variants also occur
Oral lesions tend to be chronic and may persist even after skin lesions resolve
May interfere with food intake in severe cases

5.8 Natural History

Two-thirds of patients with skin lesions clear within 1 year; many spontaneously clear in the 2nd year
Hypertrophic and mucosal disease is more chronic
Recurrences are common, particularly in older patients and those with comorbidities

6. Clinical Variants

Variant	Key Features
Linear LP	Follows lines of Blaschko (not dermatomes as originally thought); more common in children; <1% in Europeans, up to 10% in Japan
Annular LP	3-7% of LP; more common in men (90%); favors axilla, penis/scrotum, groin; ringed lesions ~1 cm; often asymptomatic
Hypertrophic LP	Verrucous plaques; anterior shins (most common site); chronic and recalcitrant; "igneous rock sign"; risk of SCC transformation
Bullous LP	Vesicles/bullae within LP lesions; distinguish from pemphigoid
Ulcerative LP	Painful ulcers on feet (rare); may be associated with cicatricial alopecia
Actinic/Subtropical LP	Photo-distributed; upper extremities and face; especially in Middle East, India, Africa
LP Pigmentosus	Diffuse dark brown macular pigmentation without typical papules; common in darker skin types
Lichen Planopilaris (LPP)	LP of hair follicles → cicatricial (scarring) alopecia; perifollicular erythema and scale; Frontal Fibrosing Alopecia is a variant
Vulvovaginal-Gingival LP	Erosive, symptomatic involvement of vulva + vagina + gingival mucosa; risk of scarring/fusion; high SCC risk
Blaschkoid LP	Follows Blaschko lines; mosaic pattern

7. Histopathology

LP is the prototype of interface (lichenoid) dermatitis - inflammation concentrated at the dermoepidermal junction.

Key histological features:

Dense, band-like (lichenoid) infiltrate of lymphocytes along the dermoepidermal junction - "hugging" the basal layer
Vacuolar degeneration / liquefaction degeneration of basal keratinocytes
Colloid bodies (Civatte bodies / cytoid bodies): Anucleate, eosinophilic necrotic basal cells (apoptotic keratinocytes) that drop into the papillary dermis; PAS-positive
Saw-tooth pattern of rete ridges: The chronic basal cell damage causes the rete ridges to assume an angulated, zigzag contour
Squamatization of basal layer: Damaged basal cells take on the appearance of spinous layer cells (response to injury)
Hypergranulosis: Prominent granular layer (in contrast to psoriasis, which shows hypogranulosis)
Hyperkeratosis (without parakeratosis - in contrast to psoriasis)
Epidermal hyperplasia (acanthosis) - chronic changes
"Max Joseph spaces": Small clefts between epidermis and dermis at the tips of some rete ridges

Feature	Psoriasis	Lichen Planus
Infiltrate	Neutrophils + T cells, perivascular	Lymphocytic, band-like at DEJ
Granular layer	Absent (hypogranulosis)	Prominent (hypergranulosis)
Stratum corneum	Parakeratosis (nuclei retained)	Orthokeratosis (no nuclei)
Rete ridges	Regular elongation ("test tubes in rack")	Saw-tooth / zigzag
Microabscesses	Munro (SC) + Kogoj (spinous)	Civatte bodies (DEJ)
Basal layer	Preserved	Vacuolar degeneration

8. Cancer Risk

Cutaneous LP alone: Not considered to carry increased cancer risk
Oral LP: ~1% risk of oral SCC; higher in smokers, alcoholics, HCV-positive patients; occurs in erosive/ulcerative LP, not reticulate
Majority of oral LP patients who develop SCC develop multiple cancers - close vigilance required
Vulvar LP: SCC risk up to 3%; regular biopsy of fixed erosive lesions essential
Hypertrophic LP: Rare SCC on lower legs
Calcineurin inhibitor use in oral/vulvar LP associated with SCC occurrence (causality not proven; monitoring essential)

9. Treatment

Topical (Mild-Moderate Cutaneous LP)

Agent	Notes
Topical corticosteroids	First-line; superpotent under occlusion for hypertrophic LP; intralesional for nail/hypertrophic LP
Topical calcineurin inhibitors (tacrolimus 0.1%, pimecrolimus)	Especially effective for oral LP; symptomatic improvement within 1 month; relapse within weeks on cessation
Calcipotriene (calcipotriol)	Equal efficacy to corticosteroids in trials
Topical sirolimus	Complete/partial remission in refractory erosive oral LP
Antihistamines	For pruritus control

Systemic (Moderate-Severe LP)

Agent	Notes
Oral prednisone	Effective for acute severe LP; minimum 15-20 mg/day; 2-6 weeks then taper
Systemic retinoids (acitretin)	Effective, especially for hypertrophic LP
Methotrexate	For recalcitrant/widespread LP
Azathioprine	Steroid-sparing
Mycophenolate mofetil	For refractory vulvovaginal LP
Antimalarials (hydroxychloroquine)	Effective in some LP subtypes
NB-UVB / PUVA	Phototherapy for widespread cutaneous LP
TNF inhibitors (adalimumab)	Reports of improvement; used in recalcitrant LP

Therapeutic Ladder (Dermatology 5e):

Level	Cutaneous	Oral	Scalp (LPP)
Topical	Corticosteroids (superpotent), calcineurin inhibitors, calcipotriene	Corticosteroids (first-line), calcineurin inhibitors	Corticosteroids, calcineurin inhibitors
Systemic	Retinoids, MTX, azathioprine, prednisone	Prednisone, MTX, retinoids, antimalarials	Hydroxychloroquine, MTX, retinoids

10. Side-by-Side Comparison: Psoriasis vs Lichen Planus

Feature	Psoriasis	Lichen Planus
Nature	Autoimmune; T-cell mediated	Autoimmune; T-cell (CD8+) mediated
Key cytokine axis	IL-23/Th17/IL-17	Th1/IFN-γ; CD8+ cytotoxic
Primary lesion	Erythematous plaque with silver scale	Violaceous, flat-topped, polygonal papule
Scale	Thick silvery (micaceous)	Scant, adherent; Wickham striae
Koebner phenomenon	Yes	Yes
Pruritus	Variable (often mild)	Often prominent; may precede lesions
Sites	Extensor (elbows, knees), scalp, sacrum	Flexor (wrists), genitalia, oral mucosa
Nails	Pitting, oil-drop, onycholysis	Longitudinal ridging, pterygium
Oral involvement	Rare	Common (~70%); reticulate white lesions
Arthritis	Yes (psoriatic arthritis in 5-30%)	No
Histology - hallmark	Munro microabscess, Kogoj pustule	Civatte bodies, saw-tooth rete ridges
Granular layer	Absent (hypogranulosis)	Prominent (hypergranulosis)
Parakeratosis	Yes	No (orthokeratosis)
Malignancy risk	Increased lymphoma risk (severe/treated)	Oral and vulvar SCC (~1-3%)
Natural history	Chronic, relapsing-remitting; rarely remits	2/3 clear in 1-2 years; mucosal more chronic
Systemic disease	Cardiovascular, metabolic syndrome	Dyslipidemia, T2DM, thyroid disease

Sources:

Dermatology, 2-Volume Set, 5th Edition (Griffiths/Barker/Bleiker) - Chapters 8, 11
Fitzpatrick's Dermatology, Volumes 1 & 2 - Chapters 12, 78
Andrews' Diseases of the Skin - Clinical Dermatology - Chapters 12 (LP), relevant psoriasis sections
Robbins & Kumar Basic Pathology - Chapter 22

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