Unusual complement activation properties of serum immunoglobulins of the pigeon Columba livia.

Complement-mediated lysis of mouse erythrocytes (MRBC) by whole pigeon antisera was found to occur in the presence of magnesium ions alone. The underlying basis for this observation was demonstrated to be the ability of IgM antibodies to activate the alternative pathway of pigeon complement, whereas IgG activates a calcium-dependent pathway possessing an unusually low lytic capacity. In addition to differing from the alternative pathway by requiring both calcium and magnesium ions, the calcium-dependent pathway exhibited higher activity at low temperature and more rapid kinetics of haemolysis. The presence of early acting C1 in pigeon serum was inferred by the selective depletion of calcium-dependent activity which occurred as a result of incubating serum containing only calcium ions with MRBC sensitized with IgG. Under the same conditions, MRBC sensitized with IgM failed to deplete complement activity, indicating that C1 does not participate in complement activation by this isotype. Interestingly, the calcium-dependent pathway detected in pigeon serum appears to more closely resemble the C1-bypass pathway rather than the classical pathway of mammalian complement.