In comparing the lymphatic tissues of sub-adult and adult shrews, young animals were generally found to have an activated immune system, as represented by a predominance of large, active, secondary follicles in the spleen and pancreas of Aselli. This suggests sub-adults were responding effectively to a diverse array of infectious agents, including the helminth and protozoan parasites detected in a number of tissues. Post-reproductive animals, however, exhibited characteristics indicative of immune system exhaustion: follicles were generally smaller and were often depleted, with a smaller proportion of secondary follicles, particularly in the spleen. This could indicate decreased follicular activity in adult animals, with impaired germinal centre reactions resulting in reduced B cell production [44] . Impairment of germinal centre reactions is a known feature of immunosenescence in vertebrates and has been studied extensively: in humans it has been shown to be a product of defective T cell-dependent B cell activation [44, 45] . Reduced lymphocyte production as a consequence of follicular and T cell impairment could explain why significantly lower numbers of white blood cells, and specifically lymphocytes, have been reported in old common shrews [46] .