Found at: doi:10.1371/journal.ppat.1000176.s001 (0.03 MB DOC) Composition and Function of Haemolymphatic Tissues in the European Common Shrew BACKGROUND: Studies of wild animals responding to their native parasites are essential if we are to understand how the immune system functions in the natural environment. While immune defence may bring increased survival, this may come at a resource cost to other physiological traits, including reproduction. Here, we tested the hypothesis that wild common shrews (Sorex araneus), which produce large numbers of offspring during the one breeding season of their short life span, forgo investment in immunity and immune system maintenance, as increased longevity is unlikely to bring further opportunities for mating. In particular, we predicted that adult shrews, with shorter expected lifespans, would not respond as effectively as young animals to infection. METHODOLOGY/PRINCIPAL FINDINGS: We examined haemolymphatic tissues from wild-caught common shrews using light and transmission electron microscopy, applied in conjunction with immunohistology. We compared composition and function of these tissues in shrews of different ages, and the extent and type of inflammatory reactions observed in response to natural parasitic infections. All ages seemed able to mount systemic, specific immune responses, but adult shrews showed some signs of lymphatic tissue exhaustion: lymphatic follicles in adults (n = 21) were both smaller than those in sub-adults (n = 18; Wald = 11.1, p<0.05) and exhibited greater levels of depletion (Wald = 13.3, p<0.05). CONCLUSIONS/SIGNIFICANCE: Contrary to our expectations, shrews respond effectively to their natural parasites, and show little indication of immunosenescence as adults. The pancreas of Aselli, a unique lymphoid organ, may aid in providing efficient immune responses through the storage of large numbers of plasma cells. This may allow older animals to react effectively to previously encountered parasites, but infection by novel agents, and eventual depletion of plasma cell reserves, could both still be factors in the near-synchronous mortality of adult shrews observed shortly after breeding. The immune system is the primary mechanism through which animals defend against parasites and pathogenic organisms. Immunity is believed to be a major factor in regulating host survival, as under natural conditions, even a mild parasitic infection may weaken an animal sufficiently to increase the chances of mortality through starvation or predation [1] . However, maintenance and up-regulation of the immune system requires energetic and nutritional resources [1] , resulting in a trade-off between investment in immunity and other physiological processes, including growth and reproduction [2] [3] [4] . An appreciation of the extent to which hosts invest in immunity is therefore critical to understanding the strategies through which animals maximise fitness, and how trade-offs are mediated between current offspring production, longevity and future reproductive success [3, 4] .