Which hypersensitivity is immune complex-mediated?

Which hypersensitivity is immune complex-mediated?

• A. Type III
• B. Type I
• C. Type II
• D. Type IV

Answer: (A)

Immune complex–mediated hypersensitivity happens when antigen–antibody complexes form and circulate, then deposit in tissues such as the kidneys, joints, or small vessels. Once stuck in these sites, they activate the classical complement pathway, producing inflammatory mediators like C3a and C5a that recruit and activate neutrophils. The neutrophils release enzymes and reactive oxygen species, causing tissue injury and inflammation. This pattern of injury is the hallmark of Type III hypersensitivity. Classic examples include serum sickness, certain forms of glomerulonephritis after infection, and the Arthus reaction in the skin. In contrast, Type I is IgE-mediated immediate allergy, Type II is antibody-dependent cytotoxic or cytolytic against cell or matrix antigens, and Type IV is T-cell–mediated delayed hypersensitivity.

Immune complex–mediated hypersensitivity happens when antigen–antibody complexes form and circulate, then deposit in tissues such as the kidneys, joints, or small vessels. Once stuck in these sites, they activate the classical complement pathway, producing inflammatory mediators like C3a and C5a that recruit and activate neutrophils. The neutrophils release enzymes and reactive oxygen species, causing tissue injury and inflammation. This pattern of injury is the hallmark of Type III hypersensitivity. Classic examples include serum sickness, certain forms of glomerulonephritis after infection, and the Arthus reaction in the skin. In contrast, Type I is IgE-mediated immediate allergy, Type II is antibody-dependent cytotoxic or cytolytic against cell or matrix antigens, and Type IV is T-cell–mediated delayed hypersensitivity.