Stable aggregated IgG (A-IgG) of various sizes, having high biological activity, were incubated at 4 degree C with adhering peritoneal macrophages from normal rats and the kinetics of A-IgG binding to the cell surface were studied. Equilibrium constants were high (2.8-11.7 X 10(8) M-1) and varied as a function of aggregate size. The maximum number of A-IgG bound per cell varied from 230,000 for A-IgG9 to 90,000 for A-IgG74. Binding was 50% inhibited by near physiological concentrations of monomeric IgC. These data suggest that A-IgG are bound at multiple sites by attachment of Fc frgments to Fc receptors present on the macrophage surface with larger A-IgG being more avidly bound. Dissociation was slower for larger A-IgG while no clear trend was seen relating associating rates and aggregate size. Thus, differences in the avidity of binding of A-IgG are due primarily to slower dissociation of larger A-IgG. Dissociationissociation of A-IgG was slower from cells exposed initially to higher doses of A-IgG and dissociation did not follow simple first order kinetics. Thus, the avidity of binding appears to be heterogeneous in a population of similar sized A-IgG. As expected, association was dose-dependent, more rapid than dissociation, and followed pseudo first order kinetics. Based on all of the above data, it is proposed that binding of A-IgG proceeds in two steps. First, A-IgG are loosely bound to perhaps a single Fc receptor. Then, depending upon the availability and mobility of Fc receptors, additional Fc fragments are attached and the A-IgG becomes more firmly attached. Thus binding is slow, but once attached A-IgG are avidly held.

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