The Macrophage Receptor for IgG: Number and Affinity of Binding Sites

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Abstract
The number of receptor sites for IgG on rabbit alveolar macrophages was determined and the strength of interaction between monomeric IgG and the cell receptors was estimated. Heavily stimulated alveolar macrophages (obtained after multiple monthly intravenous injections of complete Freund's adjuvant) possessed 2.16 ± 0.34 × 106 receptor sites for IgG per cell. Minimally stimulated cells (obtained after a single injection of adjuvant) possessed 1.21 ± 0.23 × 106 receptor sites for IgG per cell. During stimulation of macrophages with adjuvant, the increase in number of IgG receptor sites was accompanied by an increased binding of soluble immune complexes. The average association constant between these receptors and rabbit IgG was 9.02 ± 2.58 × 105 L/M and 7.59 ± 1.83 × 105 L/M, for heavily and minimally stimulated cells, respectively. Within each group the degree of adherence of the soluble complexes to the cells was directly correlated with the affinity of the receptor sites, and not with their number. Treatment of cells with proteolytic enzymes led to an increased adherence of soluble complexes. This enhanced activity was due to an increase in the average association constant of the receptor sites for IgG on enzyme-treated cells obtained from male rabbits, although no change in the number of sites was observed. Similar enzyme treatment of cells obtained from female rabbits produced an increase in the number of receptor sites for IgG, although no increase in their average association constant occurred. It is concluded that the relative adherence of soluble immune complexes to macrophages is influenced both by the number of receptor sites for IgG per cell, and by their average association constant.

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