The ability of B cells to promote autoimmune disease is dependent on their expression of the major histocompatibility complex (MHC) class II molecules required for antigen presentation to CD4+ T cells [19,36,37], expression of MHC class I molecules for activation of CD8+ T cells [22] and expression of costimulatory molecules required for effective T cell activation [21]. Here, CD4 is linked to autoimmune disease.