Immune responses of the host

Species of Eimeria differ widely in the degree of resistance to challenge induced by infection, even within the same species of host. Immunity induced in this way is species- and, in some cases, strain-specific but other methods of immunization, e.g. injection of purified antigens (above), may produce heterologous protection. The specificity, or otherwise, of immunity to the different developmental stages is not known.

Immunity is T cell mediated, apparently with little involvement of natural killer cell activity or reactive oxygen/nitrogen intermediates. Both humoral and cell-mediated immune responses participate in resistance but the latter is the more important. Antibodies are probably most effective against extracellular parasites, thereby reducing invasion. Experimentation in mice has shown that CD4+ T lymphocytes, which play the major role in controlling primary infections, are able to do so without the participation of inflammatory cells, and that CD8' lymphocytes appear to be more important in the later stages of infection. The cytokine-release profile of responding lymphocytes does not indicate any correlation between TH1 and TH2 type responses and resistance/susceptibility. Although lENy, acting via the host cell, is very effective in halting the development of the parasite, it does not seem to mediate the prompt suppression seen in immunized animals. Work in chickens indicates that this may be due to the action of CD8+ lymphocytes in preventing sporozoites from reaching their developmental site in crypt enterocytes.

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