IgG infusion induced a rapid early decline of ester-bonded C3b₂-containing complexes in patients. Plasma samples (rapidly denatured plasma) were drawn 1 hour before infusion, after infusion of 0.5 g IgG/kg body weight, and 14 days after the start of the infusion and aliquots were subjected to 2-dimensional electrophoresis. Gels were blotted and α′C3b released from ester-linked complexes was quantified using labeled mAb H206. A blot from consecutive samples of an amyopathic patient is shown. The circled letter a refers to the position of α′C3b₂-HC in the first dimension. The dotted rectangle indicates the area comprising the mAb H206-reactive polypeptides α′C3b and a fragment of α′C3b with an apparent MW of 86 kDa. The 86-kDa fragment originated from cleavage of C3b_n-containing complexes while plasma was yet in contact with cells. It was not found in serum activated by exogenous immune complexes and was also not identical with the N-terminal 65-kDa fragment, because H206 bound exclusively to the C-terminal portion of α′C3b (Figure 1 H206). The 86-kDa fragment most likely represented the large, C-terminal fragment of α′C3b, generated by factor I and CR1³⁵  before plasma was separated from cells. For quantification of released α′C3b label within rectangles was determined and normalized to pretreatment values. Results are given as means ± SD. Data taken at 0.5 g IgG/kg body weight were from 2 amyopathic (B and G) and 4 myopathic (F, H, I, and L) patients, those at 14 days from the 3 amyopathic and the myopapthic patient H treated for the first time with 2 g/kg body weight within 2 days.