Interaction of particle-bound [125I]C3b, the third component of complement, with specific receptors on human B-lymphoblastoid cells (Raji).

Taking advantage of the high density of the complex formed between the C3b receptor on cultured B-lymphoblastoid cells and particle-bound C3b, some properties of their interaction were studied. The process had an apparent dissociation constant equal to 10(-7) M. Thus particle-bound C3b has a higher affinity for C3b receptor than soluble C3b. Moreover, analysis of dissociation rate of particle-bound C3b-C3b receptor complexes suggested that a cooperative effect was induced at the cell surface by particle-bound C3b but not by soluble C3b. The most suitable explanations of these data are discussed.