Which feature best distinguishes Type II hypersensitivity (cytotoxic) from Type III hypersensitivity (immune complex-mediated) reactions?

Question

Accepted Answer

A. Direct binding of IgG antibodies to cell surface antigens with subsequent cell lysis. ## Distinguishing Type II from Type III Hypersensitivity

### Core Mechanism Difference

**Key Point:** Type II hypersensitivity involves **direct antibody binding to cell surface antigens**, whereas Type III involves **circulating immune complex deposition** in tissues.

### Comparative Table

| Feature | Type II (Cytotoxic) | Type III (Immune Complex) |
| --- | --- | --- |
| **Antibody target** | Cell surface antigens | Soluble antigens (circulating) |
| **Mechanism** | Direct IgG/IgM binding → complement activation → cell lysis | Immune complex formation → deposition in vessels/tissues → inflammation |
| **Site of injury** | Cell membrane (target cell) | Vessel walls, joints, glomeruli, skin |
| **Pathology** | Cytolysis, cell destruction | Vasculitis, arthritis, glomerulonephritis |
| **Examples** | Graves' disease, autoimmune hemolytic anemia, Goodpasture syndrome | Serum sickness, SLE, post-streptococcal GN |

### Why This Feature Discriminates

**High-Yield:** In Type II, antibodies **directly recognize and bind to cell surface epitopes** (e.g., thyroid peroxidase in Graves', RBC membrane in AIHA). This direct cell-surface binding is the pathognomonic feature and distinguishes it from Type III, where antibodies bind to soluble antigens in circulation, forming complexes that subsequently deposit in tissues.

### Clinical Pearl

**Clinical Pearl:** Type II reactions are **cell-targeted** (the cell itself is the enemy); Type III reactions are **tissue-targeted** (immune complexes rain down on innocent bystander tissues).

### Mnemonic

**Mnemonic:** **DIRECT vs. DEPOSIT** — Type II = DIREct cell binding; Type III = DEPOSITion in tissues.

[cite:Robbins 10e Ch 6]

Answer

B. Deposition of antigen-antibody complexes in vessel walls and tissues

Answer

C. Involvement of complement activation leading to membrane attack complex formation

Answer

D. Delayed onset of symptoms occurring 6–12 hours after antigen exposure

Which feature best distinguishes Type II hypersensitivity (cytotoxic) from Type III hypersensitivity (immune complex-mediated) reactions?

Explanation

Distinguishing Type II from Type III Hypersensitivity

Core Mechanism Difference

Comparative Table

Why This Feature Discriminates

Clinical Pearl

Mnemonic

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Feature	Type II (Cytotoxic)	Type III (Immune Complex)
Antibody target	Cell surface antigens	Soluble antigens (circulating)
Mechanism	Direct IgG/IgM binding → complement activation → cell lysis	Immune complex formation → deposition in vessels/tissues → inflammation
Site of injury	Cell membrane (target cell)	Vessel walls, joints, glomeruli, skin
Pathology	Cytolysis, cell destruction	Vasculitis, arthritis, glomerulonephritis
Examples	Graves' disease, autoimmune hemolytic anemia, Goodpasture syndrome	Serum sickness, SLE, post-streptococcal GN