The C′1-fixing activity of human anti-A antibodies associated with three different immunoglobulins was studied 1) by measuring the amount of free C′1 left after the incubation of whole serum with red cell-antibody complexes and 2) by C′1a fixation and transfer test. The results indicated that both polymer and monomer forms of γA-globulin antibody were incapable of fixing C′1 with the antigen. In the C′1a transfer test, it was found that the number of C′1a-fixing sites per cell was equal to the number of γM antibody molecules combined to the cell, indicating that each γM antibody molecule on a cell surface is capable of binding one molecule of C′1a, whereas at least two molecules of γG antibody are required to generate a site capable of binding a molecule of C′1a.

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