The mechanism of binding of immune complexes to macrophages was investigated using purified antibody and haptens of different valences. Antibody alone bound to macrophages; enhancement of binding occurred when polyvalent and divalent haptens were present at equivalence but did not occur in great antigen excess. Monovalent hapten did not increase the binding of antibody at any concentration ratio tried, though it inhibited the enhancement due to oligovalent hapten. Ultracentrifuged normal rabbit globulin also inhibited the binding of complexes indicating the presence of exposed binding sites on the uncomplexed molecules. Complexes bound more strongly than antibody alone as determined from elution studies. These results support the hypothesis that the enhancement of antibody binding to macrophages in the presence of antigen is due to increased energy of binding resulting from summation of individual binding sites already exposed on the antibody molecules.

It was also possible, by saturating the macrophages with gamma globulin, to estimate the number of binding sites per cell; this was calculated to be approximately 2 million per alveolar macrophage.

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