Quantitative immunology of immune hemolytic anemia. I. The fixation of C1 by autoimmune antibody and heterologous anti-IgG antibody.

The fixation of the first component of complement (C[unk]1) by rabbit and goat anti-IgG antibodies reacting with auto- or isoimmune antibodies attached to red cells has been investigated. Two molecules of the rabbit IgG anti-IgG were required to fix a single molecule of C[unk]1, whereas only one molecule of goat IgM anti-IgG was required. The relationship between the number of auto- or isoimmune antibody molecules attached to the red cells and the amount of C[unk]1 fixed by anti-IgG was determined by the concentration of anti-IgG. A concentration of anti-IgG was found such that the number of molecules of C[unk]1 fixed was directly proportional to the concentration of auto- or isoimmune antibody. By this method a sensitive, reproducible minimum estimate of the amount of cell-bound and serum antibody could be made.