Diagnosis: Leprosy (Hansen's Disease)

The image shows a large, well-defined, erythematous (reddish-pink), hypopigmented-bordered patch over the cheek extending toward the temple. The question specifies this is an anesthetic patch - loss of sensation within the lesion is a hallmark sign of leprosy.

Answer to the MCQ: D. Trigeminal nerve

Explanation

The anesthetic patch is located over the cheek/malar region of the face. This territory is supplied by the trigeminal nerve (CN V) - specifically its branches:

V2 (maxillary branch) - cheek, upper lip, lower eyelid
V1 (ophthalmic branch) - forehead, upper face

In leprosy, Mycobacterium leprae has a predilection for cooler parts of the body. On the face, the cooler regions (cheeks, nose, ears, forehead) are preferentially affected. When the facial/cheek skin patch becomes anesthetic, the trigeminal nerve is the sensory nerve involved - it provides cutaneous sensation to the face.

The facial nerve (CN VII) is a motor nerve (not sensory) and does not cause skin anesthesia when damaged; it causes facial muscle weakness/paralysis instead. The facial nerve can be involved in leprosy causing facial palsy, but it does NOT cause anesthetic skin patches.

From Fitzpatrick's Dermatology (the authoritative reference): "Besides upper and lower limbs, the face also may be affected when facial or trigeminal nerves are damaged, which can result in hypo- or anesthesia, including on the cornea" - confirming trigeminal nerve involvement in facial anesthesia in leprosy.

Key Points to Remember

Feature	Detail
Diagnosis	Leprosy (Hansen's disease)
Causative organism	Mycobacterium leprae
Hallmark sign	Anesthetic skin patch (loss of sensation)
Facial sensory nerve	Trigeminal nerve (CN V)
Why not facial nerve?	CN VII is purely motor - damage causes palsy, NOT anesthesia
Predilection for face	Cooler areas: cheeks, nose, ears, forehead

Correct answer: D. Trigeminal nerve

T cell and b cell properties for neet pg highling important ones repeatedly asked

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Now I have all the core information needed. Let me compile this into a high-yield NEET PG study guide.

T Cell & B Cell Properties - High-Yield NEET PG 🎯

1. ORIGIN & MATURATION

Feature	T Cell	B Cell
Origin	Bone marrow	Bone marrow
Maturation site	Thymus ⭐	Bone marrow (also Peyer's patches)
% of peripheral blood lymphocytes	60-80% ⭐	10-20%

⭐ Repeated MCQ: T cells = Thymus-dependent; B cells = Bursa-equivalent (bone marrow in humans)

2. SURFACE MARKERS - THE MOST TESTED TOPIC ⭐⭐⭐

T Cell Markers

Marker	Significance
CD3	Present on ALL T cells - the universal T cell marker ⭐
CD2	Sheep RBC receptor (forms rosettes) - classic T cell identifier ⭐⭐
CD4	Helper T cells; binds MHC class II ⭐⭐⭐
CD8	Cytotoxic T cells; binds MHC class I ⭐⭐⭐
CD5, CD7	Present on mature T cells
CD25	IL-2 receptor - activated T cells & Treg cells
CD45RO	Memory T cells ⭐
TCR (αβ)	Present on ~95% of T cells
TCR (γδ)	Present in skin & mucosa; NO CD4/CD8

B Cell Markers

Marker	Significance
Surface Ig (sIg)	Hallmark of B cells; mainly IgM + IgD on naive B cells ⭐⭐⭐
CD19	Pan-B cell marker (earliest marker) ⭐⭐
CD20	Mature B cells; target of Rituximab ⭐⭐⭐
CD21	Complement receptor (CR2); also EBV receptor ⭐⭐
CD22	B cell activation regulation
CD10	Pre-B cells (CALLA antigen) ⭐
CD38	Plasma cells ⭐
CD138	Plasma cells (Syndecan-1) ⭐
MHC class II (HLA-DR)	Present on B cells (antigen presentation) ⭐
CD27	Memory B cells (CD20+CD27+) ⭐

3. T CELL MATURATION IN THYMUS ⭐⭐⭐

Bone Marrow → Pro-T → Thymic Cortex → Thymic Medulla → Periphery

Stage 1 - DOUBLE NEGATIVE (DN): CD4⁻ CD8⁻
  ↓  β-chain TCR rearranges
Stage 2 - DOUBLE POSITIVE (DP): CD4⁺ CD8⁺ ← "cortical thymocyte"
  ↓  Positive selection (cortex) + Negative selection (medulla)
Stage 3 - SINGLE POSITIVE (SP):
  → CD4⁺ (Helper T)
  → CD8⁺ (Cytotoxic T)

Thymic Selection - VERY HIGH YIELD ⭐⭐⭐

Type	Where	What happens	Purpose
Positive selection	Thymic cortex	T cells that can recognize self-MHC survive	MHC restriction
Negative selection	Thymic medulla	T cells that react too strongly to self-antigens are killed	Self-tolerance
Cells failing selection	Die by apoptosis - >98% of thymocytes die ⭐

4. T CELL SUBSETS & CYTOKINES ⭐⭐⭐

Subset	CD marker	Cytokines produced	Function
Th1	CD4	IFN-γ, IL-2, TNF-β	Intracellular pathogens; macrophage activation; DTH ⭐⭐
Th2	CD4	IL-4, IL-5, IL-13	Extracellular parasites; IgE production; allergy ⭐⭐
Th17	CD4	IL-17, IL-21, IL-22	Extracellular bacteria/fungi; neutrophil recruitment ⭐
Treg	CD4, CD25, FoxP3	TGF-β, IL-10, IL-35	Immune suppression; self-tolerance ⭐⭐
CTL (Tc)	CD8	Perforin, Granzymes, IFN-γ	Kills virus-infected cells, tumor cells, allografts ⭐⭐

⭐ Classic MCQ trap: Treg cells express FoxP3 - mutated in IPEX syndrome

5. B CELL MATURATION ⭐⭐

Stem Cell → Pro-B → Pre-B (cytoplasmic μ chain) → 
Immature B (surface IgM) → Mature naive B (IgM + IgD) →
Antigen activation →
   → Plasma cell (antibody factory: IgM → class switch → IgG/IgA/IgE)
   → Memory B cell (CD20+CD27+)

Key checkpoints:

TdT (Terminal deoxynucleotidyl transferase): present in early B & T precursors; absent in mature cells ⭐⭐
First antibody produced: always IgM ⭐⭐⭐
Class switching (IgM → IgG etc.): requires T helper cell help (CD40L - CD40 interaction) ⭐⭐⭐
T-independent antigens (polysaccharides): stimulate B cells without T cell help; produce only IgM; NO memory ⭐⭐

6. COMPARISON TABLE - FREQUENTLY ASKED ⭐⭐⭐

Property	T Cell	B Cell
Antigen recognition	Peptide + MHC (TCR)	Free native antigen (BCR/surface Ig)
Receptor	TCR (cannot recognize free antigen)	Surface Ig (can bind free antigen)
MHC needed?	YES - always ⭐	NO (for B cell itself)
Products	Cytokines, Perforin, Granzymes	Immunoglobulins (IgM, IgG, IgA, IgE, IgD)
Memory	YES	YES
Rosette formation	Sheep RBC rosette (E-rosette, via CD2) ⭐⭐	EAC rosette (via CR2/CD21) ⭐⭐
Location in lymph node	Paracortex (T-zone) ⭐⭐⭐	Germinal centers / cortical follicles ⭐⭐⭐
Location in spleen	Periarteriolar lymphoid sheath (PALS) ⭐	Marginal zone / follicles ⭐

7. QUICK-FIRE HIGH-YIELD FACTS ⭐⭐⭐

CD4:CD8 ratio = 2:1 normally; reversed in HIV ⭐⭐⭐
NK cells: CD16, CD56 markers; NO CD3, NO TCR; kill without MHC recognition ⭐⭐
Nude mouse = lacks thymus → NO T cells ⭐⭐
DiGeorge syndrome = thymic aplasia → T cell deficiency ⭐⭐
Bruton's agammaglobulinemia = B cell deficiency (BTK gene mutation) ⭐⭐
Thymic hormones that promote T cell maturation: Thymosin, Thymulin, Thymopoietin ⭐
CD4 cells recognize MHC class II (think: 4 × 2 = 8; CD8 × 1 = MHC I) ⭐⭐⭐ memory trick
Largest Ig: IgM (pentamer); Crosses placenta: only IgG ⭐⭐⭐
Secretory Ig in mucosa: IgA (dimer with secretory component) ⭐⭐
CD21 is EBV receptor on B cells → EBV infects B cells ⭐⭐

8. ROSETTE TESTS - REPEATED MCQ

Test	Cell	Mechanism
E-rosette	T cells	CD2 binds sheep RBCs ⭐⭐
EAC rosette	B cells	CR2 (CD21) binds RBCs coated with antibody+complement ⭐⭐

Sources: Medical Microbiology 9e (Murray); Henry's Clinical Diagnosis and Management; Cellular and Molecular Immunology (Abbas); Fitzpatrick's Dermatology

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