The CD23 antigen is the low affinity IgE Fc receptor, which is a 45-49 kDa protein with 38 and 28 kDa fragments. It is expressed on most mature, conventional B cells (but not on peritoneal CD5+ B cells) and can also be found on the surface of T cells, macrophages and platelets. It is distinct from the high affinity IgE receptors found on basophils and mast cells which mediate allergic reactions. The low affinity receptors are thought to play a role in isotype specific immunoregulation. The regulation of CD23 surface expression appears to be integral with the complex IgE system which involves interactions of cells, cytokines, antibodies and regulatory factors. CD23 has been described as a 'membrane bound cytokine, ' in that the soluble cleavage products of CD23 are themselves able to act as cytokines in vitro.