Differentiation of Binding from Complete Activation by Use of Heterologous Components of Complement

C2hu can bind to cellular intermediates of the complement system in three forms. Native C2 fixes to the EAC4̄ intermediates presumably without alteration such that it can transfer quantitatively to a compatible EAC1̄hu4̄hu recipient cell, or can be activated on the cell by the introduction of C1̄. C2hu interaction with a homologous EAC cell forms a convertase capable of continuing the hemolytic sequence upon the addition of the terminal components. Finally C2hu can be bound by EAC1̄gp4̄gp to achieve an intermediate designated EAC1̄gp4̄gp2hhu; C2hhu differs from native C2hu by its failure to transfer and from fully activated C2hu by its failure to decay or efficiently mediate the hemolytic sequence. An EAC1̄gp4̄gp2hhu intermediate will, however, bind C3hu in a hemolytically inactive state designated C3hhu. C3hhu differs from native C3hu in that it is inactivated by the C3b inactivator and from C3b in that it fails to participate in immune adherence. Addition of C2gp to EAC1̄gp4̄gp2hhu3hhu intermediate imparts hemolytic activity and the ability to react in immune adherence. The C2gp presumably forms a homologous convertase on vacant SAC1̄gp4̄gp and in this way activates a neighboring C3hhu. C2hhu will fulfill the C2 requirements for C5 activation in the presence of C3b even though C2hhu does not fully activate C3hu and cells containing both C2hhu and C3hhu apparently do not activate C5.