Abstract
The human complement components C 1 r, C 1 s, C4, C3, factor B, and/or C{ie}1 INH were functionally blocked in normal human serum (NHS) and EGTA- or EDTA-treated NHS by polyclonal monospecific Fab'-fragments to the individual components. The results of inhibition experiments are compatible with the formation of a classical pathway fluid-phase C3 convertase {ie}(C4b2a) spontaneously generated by the inhibition of C{ie}1 INH. This process in both NHS and EGTA-NHS was accompanied by the consumption of C2, C4, C3, and factor B but only by poor enhancement of C5 conversion. Blocking subcomponent C1r, completely inhibited spontaneous activation of the complement components, indicating that the control of C1r hydrolysis is the essential role of C{ie}1 INH as a regulator of C1 activation in NHS. Noncomplement serum proteases were inactive during the initiation of the activation process. The presence of blood cells during functional inhibition of C{ie}1 INH in NHS slightly decreased the consumption of C3 but not of C2 and C4.
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