A 35-year-old woman with autoimmune hypothyroidism develops warm autoimmune hemolytic anemia (WAIHA). The Fc receptor on splenic macrophages responsible for extravascular hemolysis recognizes IgG bound to which part of the RBC-antibody complex?

• A The Fab fragment bound to RBC surface antigen
• B The complement C3b fixed to the RBC by the antibody
• C The Fc region of IgG coating the RBC surface ✓
• D The antigen-antibody complex via lectin-binding receptors

Explanation

Splenic macrophages express FcγRIII (CD16) and FcγRII (CD32) that bind the Fc portion of IgG coating the RBC surface. This triggers partial phagocytosis, removing membrane and creating spherocytes which are eventually destroyed. C3b also contributes via CR1/CR3 complement receptors on macrophages. The Fab region is antigen-binding; lectin receptors recognize carbohydrates on microbes, not IgG.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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