Activation of antibody Fc function by antigen-induced conformational changes.
IgM antibody directed against the pheny-beta-lactoside hapten was examined for its capacity to fix complement in the presence of the hapten, monohapten-substituted antigen, and multihapten-substituted antigen. Hapten was found to have no effect; monovalent antigen induced an excellent response which could be inhibited by hapten; and multivalent antigen also induced an excellent response which was related to the number of determinants added and not to the formation of antigen-antibody aggregates. The difference between the activities of hapten and monovalent antigen was reflected in their affinities for the IgM antibody. The monovalent antigen had a lower Ka, indicating that energy from binding was used to activate the Fc complement binding sites. These data show that the expression of IgM Fc function depends on a change in Fc conformation produced by the binding of antigen at the distant Fab combining sites.
D001665:Binding Sites; D001666:Binding Sites, Antibody; D003168:Complement Fixation Tests; D003165:Complement System Proteins; D006241:Haptens; D006801:Humans; D007141:Immunoglobulin Fc Fragments; D007075:Immunoglobulin M; D007700:Kinetics; D008970:Molecular Weight; D011485:Protein Binding; D011487:Protein Conformation; D012260:Ribonucleases
D006241:Haptens; D007141:Immunoglobulin Fc Fragments; D007075:Immunoglobulin M; D003165:Complement System Proteins; D012260:Ribonucleases