Hypersensitivity Reactions (Types I–IV)

Hypersensitivity reactions are exaggerated or misdirected immune responses that result in tissue injury.

Classified by mechanism:

Type I – Immediate (IgE-Mediated)

Mechanism:

Onset:

Examples:

Primary mediators:

See:

Type II – Antibody-Mediated Cytotoxic

Mechanism:

Onset:

Examples:

Key concept: 

Antibody directed against fixed tissue antigen.

Type III – Immune Complex Disease

Mechanism:

Onset:

Examples:

Key concept: 

Injury from immune complexes, not direct antibody binding.

Type IV – Delayed (T-Cell Mediated)

Mechanism:

Onset:

Examples:

Key concept: 

No antibodies involved.

Quick Comparison Table

Type Immune Driver Antibodies? Onset Example
I IgE + Mast Cells Yes (IgE) Minutes Anaphylaxis
II IgG/IgM vs cell surface Yes Hours–Days Hemolytic anemia
III Immune complexes Yes Days–Weeks Serum sickness
IV T cells No 48–72 hr Contact dermatitis

Drug Implications

Type I:

Type II:

Type III:

Type IV:

High-Yield Differentiation

If reaction occurs within minutes → think Type I.

If reaction causes cytopenias → think Type II.

If rash + arthralgias + immune complexes → think Type III.

If delayed blistering rash after drug exposure → think Type IV.