The CD40 antigen is a 44-48 kDa type I integral membrane protein of the tumor necrosis factor receptor (TNFR) superfamily. This antigen is found on B cell lines, is strongly expressed by interdigitating cells (IDC), basal epithelial cells, and is also present on macrophages, some endothelial cells, and follicular dendritic cells. It is a well-known regulator of T cell immunity including in cancer. CD40-ligand, primarily expressed on activated T cells, interacts with CD40 on APCs resulting in a ‘licensed’ state.