“Our paper provides an impetus and roadmap for studying one facet of the immune system at the cellular level in a free-living species still undergoing natural selection.”

Ecologists and evolutionary biologists have long been interested in quantifying immunity, but until recently, techniques amenable to field work or non-model animals (that is, species other than rats, mice, rabbits, etc.) were limited. The phytohemagglutinin (PHA)-induced skin-swelling technique itself has some limitations, but it has been the most powerful technique for understanding immunity in free-living species.

I feel that our paper is highly cited because it clarifies exactly what PHA is doing at the immune-cell level in one wild bird species, the house sparrow. Specifically, our paper demonstrates that many immune cell types are involved in the PHA swelling response in house sparrows, which provides insight into what changes in the degree of swelling in different contexts means. For example, we and others have found that the PHA swelling response is weakest when birds are actively breeding. By identifying what cells are inducing the swelling, we can now identify the molecular mechanisms that lead to such seasonal fluctuations in this immune response.

In part, all three are described. First, we identified what cells are present at what times during the PHA-induced immune response. Second, we provide the details necessary for others to use our technique in their own studies. Last, we synthesized all of the papers published on PHA in wild animals to highlight the abundance of information this technique has provided to ecologists.

The immune system is perhaps the most complex of all vertebrate physiological systems, but our understanding of most of the immune system comes from work on a few species (predominantly mice and rats). Our paper provides an impetus and roadmap for studying one facet of the immune system at the cellular level in a free-living species still undergoing natural selection.

Immunity is critical to almost all species at some point in their lives, but ecologists and evolutionary biologists have only recently begun asking questions about it. This technique, and others that are just now emerging, will fill this void and ideally provide fresh perspective for understanding how the immune system works.

This research was part of my dissertation at Princeton University with Martin Wikelski of the Department of Ecology and Evolutionary Biology. I became interested in how and why immune function varied in free-living birds species. Particularly, I wanted to know how the presumably greater and persistent threat of infection in the tropics influenced a bird’s immune system, compared to the more seasonal threat of infection at temperate latitudes.

Our paper emerged out of the lack of resolution of the then-available immune techniques used by ecologists. Although many problems were encountered along the way, as might be expected working in the Neotropics with a hard-to-catch species, we persevered and learned a lot about the immune systems of both populations.

I'm very interested in continuing to use an evolutionary/ecological approach to understanding the vertebrate immune system. I just accepted an Assistant Professorship position at the University of South Florida, where I will continue to study the immune systems of wild birds as well as that of rodents, the subject of my post-doctoral research.

I'm particularly interested now in how the immune system inhibits or promotes species invasions. Invasive species are one of the largest threats to biodiversity today, but we know little about the physiological characteristics of good invaders, or if indeed there are any. The immune system is a good candidate.

Information on the immune system of wild animals may greatly augment our understanding of the human immune system. At the risk of overstatement, perhaps an adaptationist perspective could illuminate why autoimmune disease is more prevalent in females than males and why allergy remains in human populations after years of selection would presumably have eliminated it. Furthermore, if we can identify immunological mediators of invasiveness, we may be better able to control, and perhaps even prevent, invasions by identifying populations or species that are most likely to be problematic.