Dynamics of an immune response in house sparrows Passer domesticus in relation to time of day, body condition and blood parasite infection

The T-cell mediated response to a challenge of the immune system with phytohemagglutinin (PHA) is a common measure used in ecological studies of host-parasite interactions or parasite-mediated selection. We investigated the temporal dynamics of this response in house sparrows Passer domesticus in order to determine factors that contribute to temporal and individual variation in PHA response. After an initial significant increase in response from injection to six hours post-injection, there was no further significant change in mean response after 12, 24, 36, 48 and 72 hours. Responses at night were consistently stronger than during daytime. Individuals may benefit from producing both a strong and a quick immune response to parasite attacks, although this may be impossible because of the observed positive relationship between maximum response and latency to maximum response. House sparrows with Haemoproteus infections had lower PHA responses and smaller maximum responses than individuals without infections. Individuals in prime body condition had stronger PHA responses than individuals in poor condition. House sparrows with large diurnal fluctuations in body mass had a weaker maximum response than individuals with small fluctuations. Since individuals with Haemoproteus infections have large fluctuations in body mass, this result suggests that infected individuals with large fluctuations in mass are unable to mount a strong immune response.