Decreased inhibition of immune precipitation by sera with the C2 B allotype

Complement-mediated precipitation inhibiting (CMPI) activity of sera of 5 individuals homozygous for C2 B was compared to that of sera of 20 individuals carrying the common C2 C allotype. Sera with the rare C2 B allotype had a depressed CMPI capacity in both the early (5 min) and the late (60 min) stages of the reaction. We have also compared the CMPI activity of seven homozygous C4A deficient (C4A*Q0) and eight C4B deficient (C4B*Q0) serum samples and did not find significant differences from the controls (no C4 null alleles) in any stage of the reaction. These results indicate that C2 is the critical component in the CMPI reaction of the two constituents of the classical pathway C3 convertase and that C2 B is less active than C2 C.