Fc-mediated immune precipitation. I. A new role of the Fc-portion of IgG.

By comparing the amounts of precipitate formed by the reaction between human serum albumin (HSA) and isomolar solutions of rabbit anti-HSA F(ab')2 fragments and of the corresponding intact rabbit anti-HSA IgG, respectively, it was found that the Fc portion of IgG was of great importance for the precipitin reaction. In the present antigen-antibody system, about half of the antigen was precipitated by this mechanism. This effect was called Fc-mediated precipitation and was most clearly expressed in the biologically important zone of low antigen excess and the zone of equivalence. It was found that the pepsin digestion did not change the ability of the F(ab')2 fragments to bind the antigen in comparison to intact IgG. Furthermore, turbidimetric analyses indicated that the Fc portion was involved in the overall mechanism of the precipitin reaction throughout the range of the precipitin curve.