Note: Library mein Apurba Sastry ki book available nahi thi, lekin yahan jo content hai woh Robbins Basic Pathology, Cellular & Molecular Immunology (Abbas), Roitt's Essential Immunology, aur Janeway's Immunobiology se liya gaya hai - jo ki standard microbiology/immunology exams ke liye equally authoritative sources hain aur Apurba Sastry ke framework ke saath align karta hai.

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MI 2.7 - Immunodeficiency Disorders

Primary (Congenital) + Secondary (Acquired including HIV/AIDS)

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PART 1: OVERVIEW - Immunodeficiency ka Basic Concept

IMMUNODEFICIENCY
       |
       |-----> PRIMARY (Congenital/Genetic)
       |         - Gene defect se hoti hai
       |         - Birth se present hoti hai (ya early childhood me manifest)
       |         - Rare hoti hai
       |
       `-----> SECONDARY (Acquired)
                 - Kisi bimari ya external factor ki wajah se hoti hai
                 - Common hoti hai
                 - Examples: HIV/AIDS, malnutrition, cancer, drugs

Primary = "Born with it" (genetic) Secondary = "Got it later" (acquired)

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PART 2: PRIMARY IMMUNODEFICIENCY DISORDERS (PIDs)

Concept Samjho - Immune System Ka Map

• B-cells → Antibody banate hain (Humoral immunity)
• T-cells → Cells ko directly kill karte hain (Cell-mediated immunity)

STEM CELL (Bone Marrow)
        |
        |-----------> LYMPHOID PROGENITOR
        |                    |
        |            --------|--------
        |            |               |
        |          B-CELL          T-CELL
        |          (Bone marrow     (Thymus mein
        |           mein mature)     mature)
        |
        `-----------> MYELOID PROGENITOR
                            |
                    Neutrophils, Monocytes,
                    NK cells, Dendritic cells

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MNEMONIC: PIDs Ko Yaad Rakhne Ka Formula

"B-Cells Are Cute; T-Cells Combine DNA Wisely"

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A. B-CELL DEFECTS (Humoral Immunodeficiency)

1. X-Linked Agammaglobulinemia (XLA) - Bruton's Disease

• Gene defect: BTK gene (Bruton Tyrosine Kinase) - X chromosome par
• BTK enzyme pre-B cell receptor ko signal nahi deta
• Result: Pre-B cell → Mature B cell mein convert NAHI hota
• Sabhi immunoglobulins (IgG, IgA, IgM, IgE, IgD) absent ya bahut kam

NORMAL:
Pro-B → Pre-B → [BTK signal ✓] → Mature B cell → Plasma cell → Antibodies ✓

BRUTON's:
Pro-B → Pre-B → [BTK mutated ✗] → BLOCK → No B cells → No Antibodies ✗

• Sirf males mein (X-linked recessive)
• 6 months tak normal (maternal antibodies protect karte hain)
• Uske baad: Recurrent bacterial infections
  • Strep pneumoniae, H. influenzae, Staph aureus
  • Sinusitis, otitis media, pneumonia, bronchitis
• Enteroviral infections bhi: Echovirus → Fatal encephalitis; Polio vaccine → Paralysis!
• Giardia persistent infection (IgA ki kami)
• T-cell mediated immunity NORMAL hai
• Lymph nodes/tonsils ABSENT ya very small (no B-cell follicles)

• All Ig absent
• No B cells in blood/lymph nodes
• T cells NORMAL

---

2. Common Variable Immunodeficiency (CVID)

• Most common symptomatic PID
• B cells hain but plasma cells mein differentiate nahi ho pate
• Low IgG aur IgA aur/ya IgM
• Both sexes affected; adult onset (20-30 years)

• Recurrent bacterial infections (same as XLA)
• Giardia infection
• Autoimmune disease risk badhta hai
• Lymphoma risk badhta hai

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3. Selective IgA Deficiency

• Most common PID (frequency mein sabse zyada)
• Sirf IgA absent; baaki Ig normal
• Usually asymptomatic (IgM aur IgG compensate karte hain)
• Kuch mein: recurrent sinopulmonary infections, Giardia

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B. T-CELL DEFECTS

4. DiGeorge Syndrome (Thymic Aplasia)

• Chromosome 22q11.2 deletion (most common deletion syndrome in humans!)
• 3rd and 4th pharyngeal pouches develop nahi karte → Thymus develop nahi hota
• Thymus nahi → T-cells mature nahi ho pati → T-cell deficiency

Chromosome 22q11.2 deletion
           ↓
3rd/4th pharyngeal pouch defect
           ↓
    ┌──────┴──────┐
    ↓             ↓
Thymus absent   Parathyroid absent
    ↓                 ↓
T-cell deficiency   Hypocalcemia →
                    Tetany (neonatal)
                         +
                    Heart defects
                    (Conotruncal anomalies:
                    Truncus arteriosus, TOF)

C - Cardiac defects (Conotruncal) A - Abnormal facies T - Thymic hypoplasia C - Cleft palate H - Hypocalcemia

• Neonatal tetany (hypocalcemia - parathyroid absent)
• Recurrent viral, fungal, intracellular bacterial infections
• Opportunistic infections (PCP, CMV, Candida)
• B-cell antibody response bhi impaired (T-cell help nahi milta B cells ko)
• Spectrum: Complete DiGeorge (rare, severe) vs Partial DiGeorge (common, milder)

• Low T cells, Low PTH, Low Ca²⁺
• Normal B cells, Normal Ig initially

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5. Purine Nucleoside Phosphorylase (PNP) Deficiency

• Toxic metabolites accumulate → Specifically T-cells destroy ho jati hain
• Similar to ADA deficiency but sirf T-cells affect hoti hain

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C. COMBINED T + B CELL DEFECTS

6. Severe Combined Immunodeficiency (SCID)

• Sabse severe PID - "Bubble Boy Disease"
• Both T cells AND B cells absent ya non-functional
• Multiple genetic causes hain:
  • Most common (50%): X-linked - yc chain (gamma chain of IL-2 receptor) mutation
  • Autosomal recessive: ADA (Adenosine Deaminase) deficiency - toxic deoxyadenosine accumulates, kills lymphocytes

yc chain (IL receptor) mutation (X-linked)
        ↓
IL-2, IL-4, IL-7, IL-9, IL-15, IL-21 ka signaling FAIL
        ↓
T-cell development block → No T-cell → No T-cell help → No B-cell activation
        ↓
SCID: Both T and B cell deficiency

ADA enzyme mutated
      ↓
Deoxyadenosine accumulates
      ↓
dATP accumulates in lymphocytes
      ↓
TOXIC to both T and B lymphocytes → SCID

• All types of infections: Bacterial, Viral, Fungal, Protozoal
• PCP (Pneumocystis jirovecii) pneumonia - classic first infection
• Oral Candidiasis
• Failure to thrive
• Persistent diarrhea
• Live vaccines contraindicated → Fatal vaccinial/polio disease ho sakti hai
• Blood transfusion se Graft-vs-Host disease → Donor T-cells host tissue attack karti hain (isliye irradiated blood use karo)

• Hematopoietic Stem Cell (HSC) Transplantation - mainstay
• Gene therapy (ADA deficiency aur X-linked SCID mein tried) - successful but some developed T-cell leukemia initially
• ADA-PEG (enzyme replacement for ADA deficiency)

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7. Wiskott-Aldrich Syndrome (WAS)

• X-linked recessive
• WASp protein (Wiskott-Aldrich Syndrome protein) mutation - involved in actin cytoskeleton remodeling
• T-cell signaling to B-cell impaired

W - Wiskott-Aldrich E - Eczema T - Thrombocytopenia + T-cell dysfunction

• Eczema
• Thrombocytopenia (small defective platelets)
• Recurrent infections (both bacterial and viral)
• IgM low, IgA & IgE elevated, IgG normal
• Poor response to polysaccharide antigens

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8. Hyper-IgM Syndrome

• CD40L (on T-cell) ya CD40 (on B-cell) mutation → No T-B interaction
• Without T-cell help → B-cell class switching NAHI ho pata
• B-cells sirf IgM banate hain, IgG/IgA/IgE NAHI bana pate

Normal:
T cell [CD40L] ←→ [CD40] B cell → IL-4/IL-13 → Class switching IgM→IgG/IgA/IgE ✓

Hyper-IgM:
T cell [mutated CD40L] ✗ → No interaction → No class switching
                                    ↓
                        Only IgM produced (very high)
                        IgG, IgA, IgE absent

• Recurrent bacterial infections (opsonizing IgG nahi)
• Pneumocystis jirovecii (CD40L also needed for macrophage activation)
• Cryptosporidium infections (intestinal)
• X-linked (CD40L mutation) - males affected

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D. COMPLEMENT DEFECTS

Complement pathway:
C1-C4-C2-C3 → C5-C9 (MAC)
     ↓              ↓
Opsonization    Bacterial lysis

C3 deficiency → Most severe → Recurrent pyogenic infections + Immune complex disease (SLE-like)
C5-C9 deficiency → Only MAC affected → Specifically Neisseria infections
C1q deficiency → SLE-like disease (immune complexes not cleared)

• C3 deficiency = Worst - all pathways affected
• Terminal complement (C5-C9) deficiency = Neisseria meningitidis aur Neisseria gonorrhoeae ki recurrent infections (esp. disseminated)

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E. PHAGOCYTE DEFECTS

9. Chronic Granulomatous Disease (CGD)

• NADPH oxidase enzyme mutation (gp91phox - X-linked; p47phox - autosomal)
• Phagocytes bacteria ko engulf kar lete hain but KILL nahi kar pate
• Respiratory burst (oxidative burst) fail hoti hai → Superoxide radicals nahi bante
• Bacteria survive inside macrophages → Chronic granuloma formation

Normal:
Phagocyte engulfs bacteria
         ↓
NADPH oxidase → O₂ → O₂⁻ (superoxide) → H₂O₂ → Bacterial killing ✓

CGD:
NADPH oxidase mutated
         ↓
No superoxide → Bacteria survive inside phagocyte
         ↓
Chronic inflammation → GRANULOMA FORMATION

• Catalase-positive organisms - because catalase destroys H₂O₂ that would have been generated by bacterial metabolism alone:
  • Staph aureus (most common)
  • Pseudomonas, Klebsiella, Aspergillus, Nocardia, Serratia
  • Mnemonic: "Cats Need Asses Pretty Quickly Somewhat" (Catalase-positive: Nocardia, Aspergillus, Pseudomonas, Klebsiella, Serratia)
• Recurrent pneumonia, lymphadenitis, liver abscess
• NBT Test (Nitroblue Tetrazolium): Normal phagocytes NBT ko blue-black karte hain; CGD mein NBT YELLOW rahta hai (no reduction)
• DHR Test (flow cytometry based) - more sensitive

10. Chediak-Higashi Syndrome

• LYST gene (lysosomal trafficking regulator) mutation
• Giant lysosomes form hote hain jo neutrophils mein granules ko degranulate nahi karne dete
• NK cell function bhi impaired

• Recurrent pyogenic infections (Staph, Strep)
• Partial albinism (melanin packaging defect)
• Peripheral neuropathy
• Giant granules in neutrophils on smear
• Accelerated phase → lymphohistiocytic infiltration (like lymphoma)

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FLOWCHART: PID Diagnosis Approach

Child with recurrent infections
           |
    ┌──────┴──────────────────┐
    ↓                          ↓
Starts < 6 months          Starts > 6 months
    |                          |
    ↓                          ↓
T-cell/SCID likely         B-cell defect likely
(DiGeorge, SCID)           (XLA, CVID)
    |
    ├── Viral, fungal, PCP → T-cell defect
    ├── All organisms → Combined (SCID)
    └── Bacteria only → B-cell defect

What type of bacteria?
  ├── Catalase+ (Staph, Pseudo, Aspergillus) → Phagocyte defect (CGD)
  ├── Neisseria → Complement defect (C5-C9)
  └── Encapsulated (Strep, H. flu, Staph) → B-cell defect (XLA, CVID)

Family history?
  ├── Males only affected → X-linked (XLA, CGD, SCID-X1, Hyper-IgM, WAS)
  └── Both sexes → Autosomal recessive (ADA-SCID, DiGeorge)

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PART 3: SECONDARY (ACQUIRED) IMMUNODEFICIENCY

Common Causes:

SECONDARY IMMUNODEFICIENCY
           |
    ┌──────┼──────────┬────────────┬────────────┐
    ↓      ↓          ↓            ↓            ↓
  HIV/  Malnutrition  Drugs    Malignancy  Other diseases
  AIDS   (most      (Steroids, (Leukemia,   (Diabetes,
        common in   chemo,      Lymphoma,    SLE,
        world)      immuno-    Myeloma)    Sickle cell,
                  suppressants)            Burns, Renal
                                           failure)

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PART 4: HIV/AIDS - Secondary Immunodeficiency

HIV Ka Pura Concept - Step by Step

Step 1: Virus Ki Structure Samjho

HIV VIRION STRUCTURE
════════════════════
Outer layer: Lipid envelope (from host cell membrane)
        ↓
Envelope proteins: gp120 (outer) + gp41 (transmembrane)
        ↓
Matrix protein: p17
        ↓
Core/Capsid: p24 (diagnostic target! - p24 antigen test)
        ↓
Inside core:
  - 2 copies of ssRNA (single-stranded positive sense)
  - Reverse Transcriptase (RT) - RNA → DNA
  - Integrase - DNA → Host genome
  - Protease - viral protein processing

• gag → Core proteins (p24, p17)
• pol → Enzymes (RT, Integrase, Protease) - drug targets!
• env → Envelope (gp160 → cleaved to gp120 + gp41) - highly variable!
• Regulatory genes: tat, rev, nef, vif, vpr, vpu

Step 2: HIV Ka Cell Mein Entry - Mechanism

HIV Entry Mechanism:
═══════════════════
                   gp120
                     ↓
            CD4 receptor (T-helper cells, Macrophages, DCs)
                     ↓
            Conformational change in gp120
                     ↓
            Co-receptor binding:
            ┌────────────────────────┐
            ↓                        ↓
          CCR5                     CXCR4
   (M-tropic/R5 strains)    (T-tropic/X4 strains)
   Macrophages, DCs          T-cells
   [Early infection]         [Later infection]
            ↓                        ↓
            gp41 fusion peptide inserts into cell membrane
                     ↓
                  FUSION and ENTRY

Step 3: HIV Replication Cycle

HIV REPLICATION CYCLE
═════════════════════

CD4+ T cell mein entry
        ↓
1. REVERSE TRANSCRIPTION: RNA → DNA (by Reverse Transcriptase)
   [NRTIs/NNRTIs block this step]
        ↓
2. INTEGRATION: DNA → Host chromosome (by Integrase)
   [Integrase inhibitors block this - raltegravir, dolutegravir]
        ↓
3. LATENCY: Proviral DNA stays dormant (can reactivate)
        ↓
4. TRANSCRIPTION: Host RNA polymerase → HIV RNA
   [tat gene accelerates this]
        ↓
5. TRANSLATION: Viral proteins made
        ↓
6. ASSEMBLY: New virions assembled
        ↓
7. BUDDING: Virus buds from cell (with host membrane)
        ↓
8. MATURATION: Protease cleaves gag-pol polyprotein
   [Protease inhibitors block this - ritonavir, lopinavir]
        ↓
Mature, infectious HIV virion released

Step 4: HIV Pathogenesis - Immunity Kaise Destroy Hoti Hai

HIV IMMUNOPATHOGENESIS
═══════════════════════

HIV enters body → Infects MUCOSAL CD4+ T cells (memory T cells)
        ↓
Spreads to lymph nodes (viremia)
        ↓
Innate + Adaptive immune response activates:
  - CTL (CD8+) response: peaks at 9-12 weeks
  - Antibody (anti-HIV) response: peaks at ~12 weeks
        ↓
But virus CANNOT be completely eliminated because:
  1. Latent reservoirs (resting CD4+ T cells with provirus)
  2. High mutation rate (reverse transcriptase has NO proofreading)
  3. gp120 highly variable → Antibodies ineffective
        ↓
Slow, progressive CD4+ T-cell depletion over years
        ↓
When CD4 count < 200 cells/μL → AIDS
        ↓
Opportunistic infections + AIDS-defining malignancies

1. Direct viral cytopathic effect
2. CTL (CD8+) cells infected T-cells ko kill karte hain
3. HIV-infected cells apoptosis undergo karti hain
4. Bystander killing (uninfected T-cells bhi kill hoti hain by inflammatory mediators)
5. HIV-infected macrophages bone marrow progenitors ko infect karte hain

Step 5: Clinical Course of HIV - "4 Stages"

HIV CLINICAL COURSE
═══════════════════════════════════════════════════════════
  CD4
count
(cells/μL)
  1000 |█ ← Acute retroviral syndrome (2-4 wks after exposure)
       |  █  Flu/mono-like illness, High viremia, CD4 drops briefly
   800 |    ██
       |      ███  ← Clinical Latency (asymptomatic phase)
   600 |         ████  ~10 years untreated (range: 2-15 years)
       |             ████  CD4 slowly declining
   400 |                 ████  Viral load low but detectable
       |                     ██
   200 |..........................██  ← AIDS (CD4 < 200)
       |                           ██  Opportunistic infections
     0 |                             █  Death
       |_________________________________________________
            Months after                  Years
            exposure     →

• 2-4 weeks after exposure
• Flu/mononucleosis-like: Fever, lymphadenopathy, sore throat, rash, myalgia, headache
• Highest viremia - most infectious period
• CD4 temporarily drops, then recovers
• Window period - antibody test negative (ELISA negative!)
• p24 antigen + NAT (PCR) can detect early

• Asymptomatic (mostly)
• CD4 slowly declining (50-100 cells/μL per year)
• Low-level viral replication continues
• Average: ~10 years (can be 2-15 years)
• Persistent Generalized Lymphadenopathy (PGL) - 2+ sites for 3+ months

• CD4: 200-500 cells/μL
• Recurrent bacterial infections, Oral candidiasis (thrush), Hairy leukoplakia (EBV), Herpes zoster, Vaginal candidiasis, Molluscum contagiosum

• CD4 < 200 cells/μL (diagnostic criterion) OR AIDS-defining illness

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AIDS-Defining Illnesses (OIs)

CD4 COUNT → OPPORTUNISTIC INFECTIONS
═════════════════════════════════════

Any CD4 level:
  - Mycobacterium tuberculosis (TB)
  - Recurrent bacterial pneumonias
  - Herpes Zoster (can occur at any CD4)

CD4 < 500:
  - Oral candidiasis (thrush)
  - Hairy leukoplakia
  - Kaposi's Sarcoma (HHV-8)

CD4 < 200:  [AIDS-defining]
  - Pneumocystis jirovecii Pneumonia (PCP) ← Most common AIDS-defining OI
  - Toxoplasma gondii encephalitis
  - Cryptococcal meningitis
  - Histoplasmosis

CD4 < 100:
  - Cerebral Toxoplasmosis
  - Cryptosporidium (chronic diarrhea)
  - Microsporidium

CD4 < 50:
  - CMV Retinitis (chorioretinitis) ← Most common cause of blindness in AIDS
  - Mycobacterium avium complex (MAC/MAI) - disseminated

1. Kaposi's Sarcoma (HHV-8) - violaceous skin lesions
2. Non-Hodgkin's Lymphoma (especially CNS NHL - think AIDS when young patient has CNS lymphoma)
3. Invasive Cervical Cancer (HPV-related)
4. Primary CNS Lymphoma (EBV associated)

---

HIV Diagnosis

HIV DIAGNOSTIC TESTS
════════════════════

SCREENING:
  4th Generation ELISA / Combo Test
  (Detects: anti-HIV antibodies + p24 antigen)
  Window period: ~2-4 weeks for combo test

CONFIRMATORY:
  Western Blot (WB) / Line Immunoassay (LIA)
  OR
  HIV RNA (Viral Load by PCR)

MONITORING:
  CD4+ T-cell count → Immune status
  HIV RNA Viral Load → Treatment response (target: undetectable)

IN NEONATES (born to HIV+ mother):
  HIV antibodies unreliable (maternal IgG crosses placenta)
  Use: HIV DNA PCR or HIV RNA PCR
  - At birth, 4-6 weeks, 4-6 months

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HIV Treatment - ART (Antiretroviral Therapy)

HIV LIFE CYCLE → DRUG TARGET
═════════════════════════════

Entry → CCR5 antagonists (Maraviroc)
      → Fusion inhibitors (Enfuvirtide/T-20)

Reverse Transcription:
  → NRTIs: Tenofovir (TDF), Emtricitabine (FTC), Abacavir
  → NNRTIs: Efavirenz, Nevirapine, Rilpivirine

Integration:
  → Integrase Strand Transfer Inhibitors (INSTIs):
     Dolutegravir, Raltegravir, Bictegravir ← Preferred in current guidelines

Maturation/Protease:
  → Protease Inhibitors (PIs): Lopinavir/r, Darunavir/r, Atazanavir

TDF + 3TC (or FTC) + DTG (Tenofovir + Lamivudine + Dolutegravir)

• All HIV+ patients regardless of CD4 count - "Treat all" policy

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SUMMARY FLOWCHART - Complete Immunodeficiency Overview

IMMUNODEFICIENCY DISORDERS
          │
    ┌─────┴──────┐
    ↓             ↓
PRIMARY        SECONDARY
(Genetic)      (Acquired)
    │               │
    │           ┌───┴────────────────┐
    │           ↓                    ↓
    │         HIV/AIDS          Others
    │         (Most imp.)       (Malnutrition,
    │                           drugs, cancer)
    │
    ├── B-CELL DEFECTS
    │     ├── XLA (Bruton's) - BTK mutation, X-linked, No B cells
    │     ├── CVID - B cells present but can't become plasma cells
    │     └── Selective IgA def - Only IgA absent, usually asymptomatic
    │
    ├── T-CELL DEFECTS
    │     ├── DiGeorge - 22q11 del, No thymus, Hypocalcemia, Heart defects
    │     └── PNP deficiency - Toxic metabolites kill T cells only
    │
    ├── COMBINED (B+T) DEFECTS
    │     ├── SCID - Severest PID, X-linked (yc mutation) or ADA deficiency
    │     ├── Wiskott-Aldrich - WASp mutation, Eczema + Thrombocytopenia + Infections
    │     ├── Hyper-IgM - CD40L mutation, No class switching, High IgM only
    │     └── Ataxia Telangiectasia - ATM mutation, Ataxia + Telangiectasia
    │
    ├── COMPLEMENT DEFECTS
    │     ├── C3 deficiency - Most severe, all bacteria
    │     └── C5-C9 deficiency - Only Neisseria
    │
    └── PHAGOCYTE DEFECTS
          ├── CGD - NADPH oxidase defect, Catalase+ bacteria, NBT test +ve
          └── Chediak-Higashi - LYST gene, Giant granules, Albinism

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RAPID REVISION TABLE - Exam ke liye

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SUPER MNEMONIC - Final Revision

S - Streptococcus pneumoniae H - Haemophilus influenzae I - Intestinal Giardia P - Poliovirus (live vaccine dangerous!) S - Staph aureus

V - Viruses (CMV, EBV, HSV) F - Fungi (Candida, PCP, Cryptococcus) I - Intracellular bacteria (TB, Listeria, Nocardia) M - Mycobacteria atypical

• CD4 < 500 → Kaposi's, Oral candida
• CD4 < 200 → PCP, Toxo (AIDS definition!)
• CD4 < 50 → CMV retinitis, MAC

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