Health impact of phytohaemagglutinin-induced immune challenge on great tit(Parus major) nestlings

Assuming that immune function is resource-limited, it can be expected to co mpete with other important functions of an organism for the total resource pool, giving rise to trade-offs in resource allocation. To test whether suc h a trade-off exists between immune defence and growth, the physiological i mpact of an immune challenge was examined in great tit (Parus major) nestli ngs, using phytohaemagglutinin (PHA) as a novel antigen. Nestlings injected intradermally with PHA in wing webs at 8 days of age produced a heterophil ic response, while their growth was not suppressed in comparison with untre ated control siblings. Nestlings that grew poorly produced a weaker cutaneo us response to PHA inoculation than well-growing nestlings. These two resul ts suggest that the response to PHA (a measure of the intensity of T-lympho cyte mediated immune responsiveness) is resource demanding, but these resou rces are not reallocated from those used for growth. This finding can be re conciled with current hypotheses, which propose that the currency in trade- offs between immune response and other demands on the organism is not neces sarily energy or macronutrients but may instead be based on immunopathology , carotenoids, or production of free radicals.