Critical role of the conversion of the third complement component C3 (beta 1C/beta 1A) for its immunochemical quantitation.

For the immunochemical quantitation of the third component of complement C3 the state of conversion of this protein in the sample and the reference serum has to be guarded. Antisera raised against C3 may show different antibody specificity for the antigenic pattern of the C3 proteins which change during proteolysis by activation and also by ageing. It is shown that anti-C3-sera gave differences from -17% to + 91% comparing fresh and aged aliquots of the same sera directly on the same immunodiffusion plate corresponding to a particular anti-C3-serum. The use of three different monospecific antisera directed against the A determinant of C3 seemed to give results with good correlation in respect to C3 conversion. Glass and polystyrene plastic tubes did not have a significantly different effect on the C3 values in respect to C3 conversion. The in vitro acceleration of C3 conversion by thrombin or an inulin-like polysaccharide did not give consistent results in this context.