Low ionic strength or chemical cross-linking of monomeric C3b increases its binding affinity to the human complement C3b receptor.

Quantitative studies of the interaction between a fragment of the third component of complement (C3b) and its receptor on human cells have been undertaken with a recently developed radioligand binding assay. Specific binding of fluid phase monomeric C3b was direct and saturable under low (mu = 0.0513) but not physiological (mu = 0.15) ionic strength. Dimeric C3b had a greater affinity for the C3b receptor at either ionic strength. Complexing of monomeric C3b with another protein (human IgG) did not result in increase in binding to human erythrocytes when compared with monomeric C3b alone. These data suggest that ionic forces are involved in C3b receptor-ligand interactions and that the stable C3b-receptor binding site is present in monomeric C3b. Furthermore they suggest that the greater affinity of dimeric or oligomeric C3b is a result of multimeric binding to preclustered C3b receptors. These findings provide an explanation for the relatively stable interaction of particle bound C3b and C3b receptor-bearing cells in the immune adherence reaction.