Isotypes of mouse IgG—I. Evidence for ‘non-complement-fixing’ IgG1 antibodies and characterization of their capacity to interfere with IgG2, sensitization of target red blood cells for lysis by complement

Abstract

The ability of IgG antibodies purified from mouse antisera to sensitize SRBC∗ or TNP.SRBC for lysis at various concentrations of guinea pig C was studied. Differences in sensitizing capacity were observed between IgG1, antibodies directed against SRBC antigens or the TNP moiety, as well as between the IgG1, and IgG2(IgG2a or IgG2b) antibodies. The IgG1, antibodies, which fix haemolytic C by classical pathway activation, could not sensitize target cells for lysis at low C levels and were found to interfere with the sensitization of cells by IgG2 antibodies. The results are consistent with the hypothesis that the loss of IgG1 lytic activity at low C concentrations and the inhibition of IgG2-mediated haemolysis by IgG1 is due to the presence of IgG1 antibodies which are unable to fix C. Since the IgG1 antibodies were structurally intact, it is concluded that two isotypes of mouse IgG1 exist, one unable and the other able to fix C upon association with antigen.