Differences between the activities of human monoclonal IgG1 and IgG3 subclasses of anti-D(Rh) antibody in their ability to mediate red cell-binding to macrophages.

Four IgG1 and three IgG3 human monoclonal antibodies specific for the blood group D(Rh) antigen were tested for their ability to mediate red cell-binding to macrophages in vitro. The IgG3 monoclonals were found to opsonize at a density of approximately 100 molecules per red cell, whereas the IgG1 antibodies were only active at a level of 10,000 molecules per cell. There was no substantial difference between the two IgG subclasses in their ability to bind to Fc receptors on macrophages and it is suggested that the more potent opsonic activity of IgG3 is the result of the relatively long hinge, leading to greater accessibility to the Fc receptor binding site on the Fc piece.