There is a striking reduction in the number of marginal zone B cells in WAS KO mice (Meyer-Bahlburg et al. 2008; Westerberg et al. 2005, 2008) and of postgerminal CD27+ B cells, the human equivalent of marginal zone B cells, in WAS patients (Park et al. 2005), which is likely associated with the reduced antibody response to polysaccharide antigens (Ochs et al. 1980). Here, WAS is linked to Wiskott-Aldrich syndrome.