Comparison of the cytophilic activities of guinea pig IgG1 and IgG2 antibodies

Monomeric guinea pig IgG1 binds to homologous peritoneal macrophages, though less effectively than IgG2. It binds to the cells with a lower association constant (Ka = 0.61 ± 0.22 × 106 L/M at 20 °C) than IgG2 (Ka = 1.44 ± 0.16 × 106 L/M), and the number of IgG1 receptor sites per cell (1.3 ± 0.18 × 106) is about one‐half that for IgG2 (2.65 ± 0.45 × 106). Enhanced IgG1 binding is observed on immune complex formation, though the increase in avidity of the complexes is dependent on their size. Marked enhancement of avidity with increase in complex size occurs only with small complexes (containing less than seven antibody molecules): with larger complexes a plateau in avidity appears to be reached. Immune complex binding is inhibited by monomeric IgG1. Comparative studies with IgG1 and IgG2 demonstrate that IgG1 is bound to the same cell type as IgG2 (primarily, the macrophage) by a nonidentical, though related, receptor.