The aim of this study was to investigate changes in selected parameters of cellular immune response in the conditions of endotoxin fever and pyrogenic tolerance in pigeons. On the first day of observation the experimental birds (n=18) were intravenously injected with Escherichia coli LPS at a dose of 10 μg/kg b.w., while the control animals (n=6) received apyrogenic physiological saline also in the form of injection. On the second and the third day of the experiment LPS was injected

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... at 24 h intervals. Four and a half hours after the saline and pyrogen administration blood samples were collected from the control and experimental pigeons. The following immunological assays were performed: WBC, leucogram and immunophenotyping of lymphocyte subsets in peripheral blood, i.e. CD 3 + (T lymphocytes), CD 4 + (T helper lymphocytes) and CD 8 + (T suppressor/cytotoxic lymphocytes) cells. In the conditions of endotoxin fever (i.e. after the first LPS injection) leucopenia, monocytopenia, heterophilia and eosinophilia were observed. Additionally, the immunophenotyping of peripheral blood lymphocytes indicated an increase in percentage of CD 3 + , CD 4 + and CD 8 + cells in response to the single injection of LPS. In contrast, the consecutive injections of LPS, which created a pyrogenic tolerance effect, caused a decrease in WBC value, heteropenia, eosinopenia and lymphocytosis. Moreover, during this state an increase in percentage of CD 3 + and CD 8 + cells was demonstrated in contrast to the percentage of CD 4 + lymphocytes. The general tendencies in cellular immune response of the affected pigeons in the conditions of endotoxin fever and pyrogenic tolerance aim at activation of defence mechanisms against LPS for its prompt elimination from the animal's organism.