The interaction between protein A (PA) and IgG immunoglobulin to Forssman antigen in solution results in the production of complexes that behave functionally like IgM in their ability to fix complement (C) and lyse sheep erythrocytes (E). Based on the results of hemolytic experiments carried out with specific anti-Forssman IgG and C, it appears that IgG not directed against the Forssman antigen can participate with specific antibody in the formation of hemolytically active (i.e., C fixing) IgM-like complexes. The ability of PA to increase the amount of 125I-labeled IgG fraction bound to E under the condition of limiting specific antibody supports this finding. Ultracentrifugation experiments carried out in a sucrose gradient and in the presence of either 125I-labeled PA or IgG was used to separate complexes formed between PA and IgG. By use of standards in these experiments, the molecular formula of the IgM-like complex was found to be [(IgG)2PA]2. Depending on the relative amounts of PA and IgG, complexes with molecular formulas corresponding to IgG2-PA and [IgG]3PA2 are also formed. These complexes may also be hemolytically active. The IgM-like complex, isolated by centrifugation is stable in terms of hemolytic activity for at least 96 hr at 4°C and at 37°C for at least 24 hr.

The ability of IgG to behave functionally like IgM in the presence of PA may account for the effects in vitro and in vivo attributed to PA interacting with IgG.

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