Species specificity in the binding of IgG to macrophages

The binding of human IgG1 and IgG3 and rabbit IgG to guinea-pig peritoneal macrophages was examined, and differences between the species, in terms of their binding mechanisms, were characterized. Rabbit IgG bound with high affinity (Kass = 3.11 +/- 0.45 x 10(6) M-1) to a finite number of receptor sites per cell (1.26 +/- 0.29 x 10(6)) and competitively inhibited the binding of guinea-pig IgG2. Heterogeneity in binding, with distinct high and low affinity components, was observed when human IgG3 was reacted with guinea-pig macrophages, while human IgG1 exhibited only low affinity binding. Neither human IgG subclass competed effectively with guinea-pig IgG2 for its cell receptor. Thus, rabbit IgG appeared to cross-react with a macrophage receptor for guinea-pig immunoglobulin, whereas the human IgG subclasses bound to macrophage membrane components that remained undefined.