Specific versus nonspecific immune defense in the bumblebee, Bombus terrestris L.

Hosts vary in both their strength of response to a general immunological insult and in their specific susceptibility to different parasite species or different strains of the same parasite. The variation in the general immune response is considered a result of the costs imposed by selection on defended individuals. The variation in the specific response may originate from variation in host and parasite genotypes and is a requirement for frequency-dependent selection. The relationship between these two fundamental aspects of defense has only rarely been studied. Using the bumblebee Bombus terrestris and its gut trypanosomal parasite Crithidia bombi we found that the host's specific response profile toward different strains correlates negatively with its level of response to a general insult. This is the opposite result one would expect if the level of general response were simply a measure of immunological quality (immunocompetence). Rather, it suggests that there is some form of a trade-off between these two fundamental aspects of the immune system. These results, therefore, shed an important light on the possible constraints that affect the evolution of the immune system and particularly the trade-off between different arms of the immune system.