Studies on complement-fixation reaction in equine infectious anemia. II. Identification of complement-fixing...

The substances responsible for inhibiting complement-fixation (CF) reaction of the late-stage serum of an equine infectious anemia (EIA)-infected horse were investigated. It was found that the IgG and IgG(T) classes in the late-stage serum were responsible for the CF inhibition. IgA could not be detected in partially purified IgG(T) by an immunodiffusion test using rabbit anti-human IgA serum. Other serum components could not be demonstrated in purified IgG by immunoelectrophoresis using rabbit anti-horse serum. The IgG class simultaneously showed CF and CF-inhibiting (CFI) activities, whereas the IgG(T) class showed only CFI activity. The IgG(T) class could exert CFI activity only when it had been reacted with the EIA antigen before addition of the reference CF serum and complement. In contrast, the IgG class converted the CF-active reference serum into a non-CF-reactive one irrespective of whether it was simultaneously reacted with the EIA antigen and the reference CF serum, whether it was added to the reaction mixture of the EIA antigen and the reference CF serum, or whether it was sensitized with the EIA antigen before addition of the reference CF serum. Inhibitory activities of the IgG and IgG(T) classes seemed to be different from each other in their reaction pattern as far as tested under our experimental conditions. Their CFI activities seemed to be specific for EIA, being negative in CFI activity in reaction with other antigens.