Reaction of an activated complex of guinea-pig complement components, C56, with unsensitized erythrocytes and with erythrocytes carrying C3b molecule.

During the interaction of guinea-pig complement intermediate cells, EAC423, with guinea-pig C5 and C6, an activated complex of C5 and C6, C56, was demonstrated in the fluid phase of the reaction mixture. C56 also was eluted from EAC42356 which had been generated by the interaction of EAC423 with C5 and C6. Both preparations of C56 showed quite similar characteristics and were not distinguished from one another. Both were capable of reacting with unsensitized erythrocytes (E) in the presence of C7 to form EC567. Further, they were able to react with EAC43 in the absence of C7 to form EAC43568 but did react with EAC43 pretreated with C3b inactivator, dithiothreitol or N-bromosuccinimide. These results indicate that guinea-pig C56 generated on EAC423 has a tendency to dissociate into the fluid phase. Nevertheless, the dissociated C56 can bind again to intact C3b molecule on the cells. The ability of cell-bound C3b to combine with C56 may lead to localization of C56 to the cell membrane carrying C3b, resulting in acceleration of attachment of C567 to the membrane. This assumption could be supported by the finding that the replacement of E by EAC43 increased the susceptibility of the cells to lytic action of complement induced by cobra venom factor. Thus, a new function of cell-bound C3b as localizing C56 to the membrane of sensitized cells was indicated.