A new paper in Proceedings of the Royal Society B: Biological Sciences contends:

I measured eight indices of immune function (haemolysis, haemagglutination, concentration of haptoglobin and concentration of five leukocyte types) in 15 phylogenetically matched pairs of bird populations from North America and from the islands of Hawaii, Bermuda and the Galapagos. Immune responses were not attenuated in insular birds, and several indices, including the concentration of plasma haptoglobin, were elevated. Thus, I find no support for the specific hypothesis that depauperate parasite communities and the costs of immune defences select for reduced immune function. Instead, I suggest that life on islands leads to an apparent reorganization of immune function, which is defined by increases in defences that are innate and inducible. These increases might signal that systems of acquired humoral immunity and immunological memory are less important or dysfunctional in island populations.

This is of course a comparison of birds, and biology is the science of exceptions...but, I have to wonder what it means for humans. In 1491 Charles C. Mann reports that some researches in the field of immunology suggest that not only do the indigenous populations of the New World exhibit reduced MHC polymorphism vis-a-vis Eurasian populations (MHC is an extreme case of balancing selection where overdominance and frequency dependent factors seem to perpetuate the maintenance ancient lineages), but, they seem to have specialized defenses against larger parasites (as opposed to microbial pathogens). In other words, not only are they depauperate in their immune system, but, they might have developed different responses due to local selective pressures, and lack of the meta-populational pool which periodically bathes Eurasia in novel plagues. This has implications for insular groups like Andaman Islanders, they might be well adapted in regards to natural selection, but their adaptive immune system might be rather primitive in comparison to Eurasians who have served as the ideal habitats for virulent plagues over the past 10,000 years.