A second pregnancy ends with delivery of a jaundiced neonate who develops kernicterus. The mother is Rh negative and was never given anti-D prophylaxis. The infant's direct Coombs test is positive. Which hypersensitivity mechanism explains the haemolysis?

• A Type I, IgE-mediated mast cell degranulation
• B Type II, IgG-mediated opsonisation of fetal red cells ✓
• C Type III, immune complex deposition in fetal tissues
• D Type IV, CD4+ T-cell mediated tissue injury

Explanation

Rh-positive fetal red cells sensitize an Rh-negative mother, who produces IgG anti-D. In subsequent pregnancies these maternal IgG antibodies cross the placenta, coat fetal erythrocytes, and cause their destruction by splenic macrophages: antibody-dependent cellular cytotoxicity, a classic Type II (cytotoxic antibody) reaction. The positive direct Coombs test confirms antibody coating of the infant's cells. Option C fails because Type III reactions involve soluble antigen-antibody complexes depositing in vessels, whereas here antibody binds directly to fixed cellular antigen.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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