Abstract
Anti-C3d [complement component 3d] sera were raied to different [human] C3d antigens or to the same C3d antigen by different methods. Athough identical by immunoprecipitation studies, the various anti-C3d sera showed differences in specificities against bound C3d antigen. Such differences were observed with red blood cells coated with C3d in vivo and in vitro. Antisera made to the C3d-KAF [C3d bound to C3b inactivator-treated complement coated erythrocytes] antigen detected fewer molecules/cell on C3d-tryp [trypsin treated complement coated erythrocytes] cells than did antisera made to the C3d-tryp antigen. The converse was true for C3d-KAF cells. Saturation experiments indicated that different anti-C3d detected different bound C3d subpopulations. C3d bound to red blood cells in vivo was, in at least 1 case, detectable by some anti-C3d sera but not by others. Such differences in anti-C3d specificity may be important in determining the optimal characteristics of anti-C3d antiglobulin serum for routine laboratory use.