Charge and size heterogeneity of C3d following in vivo and in vitro activation of the complement system.

Split products of the third complement factor (C3) expressing D but not C epitopes (C3d) were analyzed by crossed immunoelectrophoresis and size chromatography. Four molecular forms (termed 1, 2, 3, and 4 from the anodic side) were identified. The precipitation pattern of C3d in serum following acute in vivo activation was similar to the patterns observed using in vitro activation by MgCl2, zymosan, Escherichia coli, and delta IgG. These patterns were different from those observed in normal human serum and serum from a patient suffering from systemic lupus erythematosus. The molecular weights of forms 1 and 4 were approximately 170,000 daltons and those of forms 2 and 3 approximately 40,000 daltons.