Phagocytosis of C3b-coated sheep red blood cells by guinea-pig macrophages in the absence of antibody.

C1 was passively attached to tannic-acid-treated SRBC in the absence of antibody. EC1,4,2,3 were prepared by the stepwise reaction of EC1 with C4, C2, and C3. Contamination with IgM or IgG antibodies of the complement components used could not be detected by immunochemical methods; furthermore, possibly contaminating traces of antibody were removed by careful absorption of all preparations with erythrocytes. The C3b-opsonized SRBC were ingested by unstimulated guinea-pig peritoneal macrophages and PMN leukocytes. Addition of purified IgM to the C3b-opsonized SRBC did not substantially enhance particle uptake. It is concluded that C3b alone can act as an opsonin and triggers phagocytosis in the absence of antibody.