Bovine IgM: Does it fix guinea pig complement in the absence of bovine complement components?

Purified bovine isotypes IgM, IgG1, IgG2, and IgA (secretory), affinity purified with Brucella abortus, were tested in a complement fixation test (CFT) for their ability to activate guinea pig complement directly or in the presence of 'normal' bovine serum. Only IgG1 fixed guinea pig complement in the direct test and approximately 250 ng of antibody was required to activate 50% of 3 CH50 units with a standard amount of antigen. Addition of 'normal' bovine serum as an additional source of complement resulted in activation of guinea pig complement by IgM, IgG2 and secretory IgA at levels of approximately 1200, 700 and 2250 ng, respectively for 50% of 3 CH50 units. Addition of 'normal' bovine serum did not enhance complement activation by bovine IgG1.