Crossed immunoelectrophoretic analysis of split products of the third complement factor (C3d) following in vivo activation of the complement system

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Abstract
Split products of the third complement factor expressing D but not C epitopes (C3d) were analysed by crossed immunoelectrophoresis using the intermediate gel technique. Four molecular forms of C3d were identified and designated 1, 2, 3 and 4 according to their electrophoretic migration velocity. Whereas the relative concentration of molecular forms 2 and 3 were increased following acute in vivo activation and nephritic factor (NeF)‐mediated in vitro activation, the relative concentration of molecular form 1 was found to be increased in patients with chronic activation of the complement system. The results obtained by the addition of anti‐C3c antibodies to the first‐dimensional gel suggested that form 4 was a C3d‐like molecule split from the native C3 during electrophoresis. By the addition of anti‐albumin antibodies to the first‐dimensional gel it was demonstrated that molecular form 1 of C3d was a complex between C3d and albumin. The immunoprecipitation patterns of molecular forms 2 and 3 was unaffected by anti‐albumin and anti‐C3c antibodies added to the first‐dimensional gel. These data suggest that molecular forms 2 and 3 of C3d are the primary split products of C3b and these molecules are able to form a stable complex with human albumin.