Heterophil/lymphocyte ratios predict the magnitude of humoral immune response to a novel antigen in great tits (Parus major).

Animals display remarkable individual variation in their capacity to mount immune responses against novel antigens. According to the life-history theory, this variation is caused by the costs of immune responses to the hosts. We studied one of such potential costs, depletion of somatic resources in wintering wild-caught captive passerines, the great tits (Parus major) by immune challenging the birds with a novel antigen, killed Brucella abortus (BA) suspension. We found that despite mild temperature conditions in captivity and ad libitum availability of food, immune challenge depleted somatic resources (as indicated by a body mass loss) and elevated relative proportion of heterophils to lymphocytes (H/L ratio) in the peripheral blood of birds. However, body mass loss did not covary with an increase in H/L ratios between two sampling events, which indicates that these two markers of health state describe different aspects of individual physiological condition. Antibody titres were not associated with the extent of body mass loss during the development of immune response, which shows that the somatic cost of immune response was not proportional to the amount of antibody produced. Birds with high pre-immunisation H/L ratios mounted weaker antibody response, which is indicative of stress-induced suppression of humoral immune response and is consistent with the concept of an antagonistic cross-regulation between different components of the immune system. The latter finding suggests a novel diagnostic value of H/L ratios, which reinforces the utility of this simple haematological index for prediction of the outcomes of complicated immune processes.