Wiskott-Aldrich syndrome (WAS) is an X-linked immunodeficiency disorder characterized by eczema, recurrent infections, thrombocytopenia and small platelets. There is an increased incidence of autoimmune phenomena particularly autoimmune haemolytic anaemias and vasculitic disorders. Mutations in the WASP gene encoding the cytoskeleton regulatory protein WASp (Wiskott-Aldrich syndrome protein) result in abnormal protein activity with defective cytoplasmic signaling and actin polymerization. This accounts for abnormal T cell responses to proliferation and susceptibility to infections, but does not fully explain the autoimmune phenomena nor the progressive lymphopenia seen in these patients. Wiskott Aldrich patients also demonstrate abnormal O-glycosylation of a highly conserved transmembrane glycoprotein CD43 that is expressed on most haemopoeitic cells. The altered glycosylation pattern on WAS lymphocytes is due to increased beta1-->6 GlcNACtransferase activity which leads to branched core 2 glycans or lower molecular forms of CD43 glycoprotein. The clinical hypothesis put forward is that abnormal O-glycosylation of CD43 may underlie the development of the autoimmune disorders and the progressive lymphopenia observed in WAS patients. Regulation of glycosylation of CD43 is important in the selection process of T cells within the thymus and abnormalities of glycosylation may cause many immune perturbations, such as the escape of self-reactive T cells into the periphery and subsequent development of autoimmune disease in these patients.