CD40, belonging to the TNF receptor family, is a cell surface protein generally expressed on B cells, dendritic cells, monocytes, thymic epithelial cells and on T cells at low levels. The membrane-anchored CD40 Ligand is expressed almost exclusively on activated CD4+ T lymphocytes. CD40 signaling plays a significant role immunoglobulin (Ig) class switching, and is critical for the proliferation and differentiation of B cells. A disease "immunodeficiency with hyper-IgM", characterized by failure to produce IgG, IgA and IgE due to failure expression of CD40L. The soluble form of CD40L is produced in vivo by an intracellular proteolytic processing of the entire TNF homologous region (18 kDa protein) of CD40L.