C3 Consumption in Immune Reactions

C3 consumption and site formation was studied with specific antisera against three antigenic determinants of the C3 globulin molecule, namely, [β 1 C] or B, the β 1 A and the α 2 D determinant. Interaction between C3 in serum and optimal specific precipitates results in the rapid loss of the [β 1 C] or B determinant and concomitant appearance of C3i and free α 2 D in the fluid phase, the latter determinant being nearly unexposed in intact C3 globulin. Experimental data further suggest that the B determinant of the intact C3 molecule is lost by destruction or by intramolecular rearrangement in the C3 molecule after interaction with cell bound C. This is borne out by the following data. (a) Immune aggregates with bound complement are incapable of inducing anti-[β 1 C] antibodies in rabbits; (b) sensitized Le a -positive red cells with bound complement agglutinate with monospecific anti-α 2 D and anti-β 1 A, but fail to agglutinate with anti-[β 1 C]; (c) EAC1, cells prepared with limited C3 fail to agglutinate with anti-[β 1 C] but agglutinate strongly with anti-β 1 A and anti-α 2 D.