Initiation and Regulation of Complement during Hemolytic Transfusion Reactions

Figure 2

C3 degradation following complement activation. Conversion of C3 to C3b by C3 convertase complex results in rapid deposition of highly reactive C3b on the target surface. Degradation by inhibitory factor I, in concert with other regulatory molecules, results in conversion of C3b to iC3b. Although iC3b retains the capacity of C3b to engage complement receptors as an opsonin, it no longer participates in the production of additional complement. Additional cleavage of iC3b by factor I results in the formation of C3d, which no longer fixes complement or serves as an opsonin, although it may engage B cells and facilitate antibody formation. Decoration of antigen with C3d may result in protection of the antigen from additional antibody binding and subsequent effector function.