PERSPECTIVE: Principles of ecological immunology

Studies of immune mechanisms and immune function are widespread in modern biology. Furthermore, studies of the immune system have been central to the fields of health and medicine for over a century (Silverstein 1989). As a result, the immune system of vertebrates and in particular that of model organisms, such as mice, has been described in great detail (for an introductory yet comprehensive overview see Delves et al. 2006). These studies have tended to be with lab-based models in controlled conditions. While extremely useful, an understanding of immunity in the context of natural environments was lacking, until in the mid-1990s the field of ecological immunology emerged (Sheldon and Verhulst 1996). Ecological immunology went beyond outlining the physiological or molecular basis of immune responses, by placing them in the context of ecology and adaptation. In fact, despite the obvious benefits of fending off parasites (throughout this article parasite is used in its evolutionary sense, including viruses, bacteria, fungi, protozoa and metazoan parasites), hosts remain susceptible and immune responses vary widely across species and situations. Thus, a principal aim of ecological immunology is to understand variation in parasite resistance and immune responses. In the years following its inception, ecological immunology has expanded rapidly and now boasts a huge number of studies investigating topics such as costs associated with immunity and the optimal use of immune defence. While the initial ideas that sparked the field were relevant for vertebrates, and in particular birds (Folstad and Karter 1992; Sheldon and Verhulst 1996; Norris and Evans 2000), subsequent work has been carried out on a broad range of taxa, with invertebrate ecological immunology being a particularly fruitful area (Rolff and Siva-Jothy 2003). The ideas and concepts at the base of ecological immunology are of great importance for the areas of health and medicine. They allow us, for example, to understand variability in immune responses between populations, to relate the outbreak of diseases to the evolutionary history and ecology of populations in humans and economically important animals, and to analyze vectors of human and livestock diseases (for example mosquitoes and malaria, Tripet et al. 2008). In addition, there are a great number of other studies where results cannot be directly applied to health and disease in humans or domestic animals, but where the general concepts that they address are