Fab fragments of a murine C3d-specific monoclonal antibody, clone 4C2, strongly inhibited the binding of H, and up to about 80% that of B, to C3b on sheep erythrocytes, as shown by using radiolabelled purified proteins. The inhibition was also detected in the fluid phase. Conversion of C3 by preformed solid phase alternative pathway C3 convertase was not affected by 4C2. Also, it did not affect the decay rate of the convertase, but reduced the decay-accelerating effect of H on it. The results suggest that the binding sites of B and H on C3b are close to each other, and their competition for binding to C3b is focused around the C3d region. In addition, 4C2 partially blocked noncovalent binding of C3b to human erythrocytes, which suggests that CR1 also binds close to the H- and B-binding region in C3b.

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