The effect of carbamylation or amidination of rabbit γG-antibody on its ability to fix guinea pig complement has been studied. These procedures, which result in the conjugation of lysine groups, cause a reduction in complement-fixing ability without significant loss in the antibody's ability to combine with antigen. Although heavy carbamylation can reduce the ability of antibody to precipitate antigen, this effect was not observed with amidination. At least a 55% loss in fixing ability, produced by light conjugation by means of either carbamylation or amidination, is probably due to the inactivation of specific complement-fixing sites; with increasing degrees of conjugation, further loss in fixing ability results, due to nonspecific alterations of the antibody molecule. These losses are associated with, but not caused by a diminution in the rate of antigen-antibody aggregation.

Preformed complexes of antigen and unconjugated antibody are less effective at fixing complement than immune complexes formed in the presence of complement. Both amidination and carbamylation abolish this difference.

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