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

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 IgG 1 , antibodies directed against SRBC antigens or the TNP moiety, as well as between the IgG 1 , and IgG 2 (IgG 2a or IgG 2b ) antibodies. The IgG 1 , 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 IgG 2 antibodies. The results are consistent with the hypothesis that the loss of IgG 1 lytic activity at low C concentrations and the inhibition of IgG 2 -mediated haemolysis by IgG 1 is due to the presence of IgG 1 antibodies which are unable to fix C. Since the IgG 1 antibodies were structurally intact, it is concluded that two isotypes of mouse IgG 1 exist, one unable and the other able to fix C upon association with antigen.