Why does administering mismatched packed red blood cells (RBCs) expressing a foreign antigen result in rapid intravascular hemolysis?

💡 Explanation

Pre-existing antibodies, such as anti-A or anti-B, in the recipient's plasma bind to the foreign antigens on the transfused RBCs, thereby activating the classical complement pathway, causing direct lysis of the RBCs. Therefore, rapid intravascular hemolysis occurs, rather than delayed mechanisms involving T-cell cytotoxicity or ATP reduction.

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