Composition and Function of Haemolymphatic Tissues in the European Common Shrew

Bray, Daniel P.; Bennett, Malcolm; Stockley, Paula; Hurst, Jane L.; Kipar, Anja

It has been suggested that short-lived species should limit their investment in immunity to immediate, innate responses, as the energetic costs associated with mounting specific immune reactions are unlikely to be outweighed by the benefits of increased long-term survival [1] . The dependence on innate responses may be greater for species with limited energetic reserves (such as S. araneus), as even a mild immune challenge is likely to result in starvation if allowed to persist for more than a short time [1] . Here, however, the presence of numerous active secondary follicles in the spleen and pancreas of Aselli, the development of small lymphatic follicles in portal areas in the liver and the generally high number of plasma cells in the pancreas of Aselli all indicate that common shrews remain consistently able to mount systemic, specific immune responses. We also observed macro- phage-dominated (granulomatous) inflammatory reactions with lymphocyte involvement in both sub-adult and adult shrews, which included reactions to helminths in tissues. The increasing number of plasma cells in the medulla of the pancreas of Aselli and in lymph nodes with advancing age might even suggest a 'refocusing' of the immune system, from reacting to novel antigens in follicles as a young animal, to combating previously experienced parasites or pathogens with appropriate antibody responses as an adult. Plasma cells are long-lived and can survive for weeks after immunisation, particularly when not too tightly packed [50] ; perhaps young common shrews invest in long term immunity by producing and storing plasma cells in the pancreas of Aselli, which can then be used to mount efficient responses against previously encountered parasites in adulthood, when reproduction places greater demands on internal resources [8] . While this strategy may allow older animals to react effectively to previously encountered parasites, infection by novel agents or eventual depletion of plasma cell reserves, could still be factors in the near-synchronous mortality of adult shrews observed shortly after the breeding season [12, 13, 18]