Absence of CR1 receptors on red blood cells would primarily reduce the clearance of immune complexes by which organ?

• A. Decreased binding of C3b to RBCs
• B. Decreased clearance of immune complexes by the spleen
• C. Decreased breakdown of C1qrs
• D. Decreased binding of Factor H

Answer: (B)

Red blood cell CR1 (CD35) binds immune complexes coated with C3b/C4b and acts as a shuttle, carrying these complexes to phagocytes in the spleen (and to a lesser extent the liver) for removal. If CR1 on RBCs is absent, this clearance pathway is impaired, so the spleen loses its primary means to clear circulating immune complexes. The other processes—such as breakdown of C1qrs or regulation by Factor H—aren’t the main route by which immune complexes are removed from circulation, and the binding step mediated by CR1 on RBCs directly supports the spleen’s role in this clearance.