Bactericidal activity of meningococcal antisera. Blocking by IgA of lytic antibody in human convalescent sera.

Humoral IgA, separated and purified from human sera drawn 12, 27, and 33 days after infection with serogroups B, Y, and C meningococci, respectively, has been shown to inhibit complement-mediated bacteriolysis by IgG and IgM purified from the same sera. Inhibition was dependent on the ratio of lytic to blocking antibody, was strain specific, and was greater for IgG than IgM. IgM was found to be more active than IgG in a bactericidal test. Activity of IgA appeared unrelated to the presence of a prozone in the whole serum. Similar inhibition was noted for IgA separated from rabbit sera.