A patient has cold agglutinin hemolytic anemia. The antibody class responsible and the complement pathway primarily activated are:

• A IgM; classical pathway activated by IgM bound to RBC surface antigens at cold temperatures ✓
• B IgG; classical pathway via C1q binding
• C IgA; alternative complement pathway
• D IgE; lectin pathway

Explanation

Cold agglutinin disease is caused by IgM autoantibodies (typically against the I/i antigen on RBCs) that bind RBCs at temperatures below 37°C (peripheral extremities in cold weather). IgM efficiently activates the classical complement pathway by binding C1q, leading to complement-mediated hemolysis and/or agglutination. As blood warms centrally, IgM may dissociate but complement fragments (C3b) remain, causing extravascular hemolysis via splenic macrophages. Cold agglutinin disease is associated with Mycoplasma pneumoniae infection, infectious mononucleosis, and B-cell lymphomas.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.