Host parasite coevolution assumes pathogen specific genetic variation in host immune defense. Also, if immune function plays a role in the evolution of life history, allocation to immune function should be heritable. We conducted a cross-fostering experiment to test the relative importance of genetic and environmental sources of variation in T-cell mediated inflammatory response and antigen specific antibody responses in the great tits Parus major. Cell mediated response was measured during the nestling period and antibody response against two novel antigens was measured in two-month-old juveniles raised in a laboratory. We found no effect of nest of origin, but a strong effect of rearing environment on cell mediated response. In contrast, we found a large effect of nest of origin on antibody response to both, diphtheria and tetanus antigens suggesting genetic variation. In a model where responses to both antigens were analyzed simultaneously, we found a significant origin-by-antigen interaction, suggesting that genetic variation in antibody responses is specific to particular antigens. Large genetic variation in antibody responses found in this study suggests that host immune defense may evolve and specificity of genetic variation in antibody responses suggests that host defense may be pathogen specific as models of host-parasite coevolution suggest. Our results also suggest that different immune traits are to some degree independent and outcome of the interactions between immune function and the environment may depend on the particular immune trait measured.