The role of secretory IgA in anti-coccidial immunity in the chicken.

The serological and secretory immune responses of the chicken to infection with Eimeria tenella were evaluated in terms of various anti-coccidial activities. Serological responses were detected in the forms of precipitating, sporozoite neutralizing, anti-merozoite and anti-schizont antibodies. Similarly, anti-schizont and sporozoite neutralizing activities were found in caecal contents (containing mainly IgA) from infected birds and these also had the capacity to damage second generation merozoites. Moreover, the functional importance of IgA could be implied from the substantial predominance of IgA synthesizing cells in the intestinal immunocyte response as revealed by immunohistology. This was reflected in the immunoglobulin profile of caecal contents, for primary and secondary infection resulted in elevated levels of IgA whilst IgG and IgM generally remained extremely low or were usually undetectable. Taken with the well established lack of correlation between serum antibody and protection, these results suggest that the intestinal secretory IgA system plays an essential role in the protective immune response to E. tenella.