On the interaction of the first complement component C1 and its subunit C1q with solid-phase IgM immune complexes.
The interaction of C1 and C1q with solid-phase anti-dextran MOPC-104E IgM was studied. An enzyme-linked immunosorbent assay (ELISA) was used to detect bound C1q. The results revealed that immobilized IgM is converted to the functionally active 'staple' conformation by the specific polyvalent ligand dextran (B 1355/S). C1q is fixed to IgM dependent on the antigen concentration, and its binding might be explained by assuming a functional binding constant (K) of approximately 10(9) M-1. Molecules bound with a K in the range of 10(7) M-1 cannot be detected by this ELISA procedure. The fixation of C1q saturated with an excess of the C1r2S2-tetramer differs from that of free C1q. C1q incorporated in the C1 complex rapidly dissociates independently of the antigen concentration. Since the complement binding sites are located at definite positions on the IgM molecule because of its pentameric structure, it is suggested that the distinguishable association properties of C1 and C1q are brought about from the altered flexibility of the C1q molecule complexed with C1r2S2.