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1. Knowledge of how pathogens activate different components of the immune system to combat infections in the wild is a first step towards building a more global picture of the role that immunocompetence plays in host-parasite evolution and host life history. Ecological immunology is currently situated in this first step due, in part, to the scarcity of simple techniques to reliably evaluate immunocompetence in the field. In fact, controversy remains as to whether parasites act as immuno-stimulators promoting immune responses or as immune-depressors via immune resource depletion. 2. In this study we manipulated parasite infection ( Caryospora, Protozoa, Apicomplexa) and vaccinated captive Eurasian kestrels ( Falco tinnunculus) to study potential immunological responses in different components of the immune system: circulating lymphocyte lineages (CD4, CD5 and CD8), counts of total lymphocytes and white blood cells (WBC) and plasma immunoglobulins (α-1, α-2, β-1, β-2 and δ). 3. We also evaluated the efficacy of the PHA-assay, a common test used in ecological immunology, to gain information about T-cell-mediated immune response. 4. Experimentally infected and vaccinated (inoculated with attenuated parasites) birds showed drastic increases in CD4 and CD5 lymphocyte lineages, but not in CD8s, nor in total counts of lymphocytes or WBCs and no significant variation was observed in plasma globulins and in inflammatory reaction to PHA as a consequence of infection. PHA assay values were not correlated with initial (before vaccination) or final (after vaccination) values or with the response (final – initial) of T-lymphocyte lineages. 5. Individuals with initially lower numbers of CD4 showed higher numbers (higher immune response) after vaccination. In the case of CD5s, initial and final numbers were positively correlated. 6. This study shows that parasites clearly provoke immune activity stimulation through proliferation of T-lymphocytes (CD4 and CD5) and also reveals that other commonly used measurements, considered to be related to T-cell-mediated immunity, such as single-time-point measurements (total lymphocyte and WBC counts) or PHA assays, have a questionable capacity to provide information about immunological capacity to combat pathogens. [ABSTRACT FROM AUTHOR]