The C4 and C2 but not C1 components of complement are responsible for the complement activation triggered by the Ra-reactive factor.

Ra-reactive factors (RaRF) are the name of a group of C-dependent bactericidal factors that bind specifically to Ra chemotype strains of Salmonella. These factors are present in the sera of a wide variety of vertebrates and have common characteristics. Here we investigate the C components required for the C activation induced by mouse RaRF, by using hemolysis of Ra LPS-coated E (ELPS) as a model system. It was found that C1-depleted and C1q-depleted sera were as effective as the undepleted serum in the lysis of ELPS sensitized with RaRF. Addition of the C1 component or C1q subcomponent to the depleted sera did not increase the effect. On the other hand, C4 and C2 components were found to be essential for the lysis of RaRF-sensitized ELPS. Activities of C4 and C2 remained on the sensitized cells even after washing the cells, suggesting that the classical C3 convertase, C4b2a, is generated on the RaRF-sensitized ELPS.