The Complement-Fixation Reaction in Equine Infectious Anemia: Demonstration of Inhibition by IgG(T)

The role of non-complement-fixing anti-equine infectious anemia (EIA) antibody in the conversion of complement fixation (CF) tests from positive to negative in EIA-infected horses was investigated. Complement-fixation inhibition (CFI) tests demonstrated antibodies in sera that were CF negative. These antibodies would bind to antigen, but would not fix complement. The inhibiting antibodies were isolated and shown to be IgG(T) by immunoelectrophoresis and immunodiffusion against monospecific anti-IgG(T) antisera. Separation of immunoglobulins from affected horse sera by DEAE cellulose chromatography revealed 11 of 13 had complement-fixing IgG and non-complement-fixing IgG(T) antibodies present simultaneously regardless of the reactivity of whole sera. The reactivity of whole sera in CF and CFI tests evidently depended upon the relative concentration and perhaps affinity of the Ig classes. The predominant class fluctuated in serial sera from four of six persistently infected horses.