How does presence of parasites influence the evolution of immune function in wildlife?

Immune function in a critically important component of an organism’s life history: it literally ensures the survival of an individual and ultimately of a population and species. While biologists have a detailed understanding of the function of the human immune system, we have only a vague understanding of how the immune systems of other vertebrates work. One fundamental question is to which extend does the environment and in particular the number of parasites and pathogens that an individual encounters, affect immune function. 

This study, lead by Karin Lindstrom and J. Foufopoulos, took advantage of different epidemiological environments in four populations Darwin’s finches on the Galapagos to compare parasite load and immune system makeup. Two predictions were made: first, populations on smaller islands would harbor relatively fewer parasites, and second that because of the reduced burden of infection immune function would be accordingly down-regulated. Parasitological surveys on the focal islands supported the first prediction: finches on smaller islands had fewer wing and crown mites, as well as reduced rates of avian pox. However immune function changed in monotonic but counter-intuitive ways: while antibody production followed the predicted pattern (stronger on larger islands), cell-mediated responses followed the opposite pattern declining from smaller to larger islands. This suggests that there are potential trade-offs between different arms of the immune system and that different strategies are better for parasite-poor vs. parasite-rich environments. (see Lindstrom et al. 2004, O'Connor et al. 2005, and Lindstrom et al. 2009).