CD40, also known as TNFRSF5, belongs to the tumor necrosis factor receptor superfamily (TNFR-SF) and is a type I transmembrane glycoprotein with a molecular weight of 45-50kD. CD40, also known as TNFRSF5, belongs to the tumor necrosis factor receptor superfamily (TNFR-SF) and is a type I transmembrane glycoprotein with a molecular weight of 45-50kDa. It contains 173 amino terminal AA extracellular regions, transmembrane regions, and 62 AA cytoplasmic regions. Like other members of the TNFR family, the extracellular segment of CD40 molecule has four serine rich amino acid repeat sequences (CRDs1 CRDs4), and the regions that bind to CD40L are mainly CRD2 and CRD3. CD40 is mainly expressed in B cells, thymic epithelial cells, activated monocytes/macrophages, dendritic cells, hematopoietic cells, epithelial cells, endothelial cells, and some tumor cells (such as ovarian cancer, nasopharyngeal cancer, bladder cancer, liver cancer, adenocarcinoma, Hodgkin's disease, chronic lymphocytic leukemia, non Hodgkin B lymphoma, multiple myeloma, non-small cell lung cancer). In the thymus, CD40 is expressed on the epithelium of the cortex, medulla, and thymic stromal cells. These cells continuously express CD40 at low levels in a non diseased state, and are affected by inflammatory cytokines such as IL-1, IL-3, and TNF- α、 GM-CSF, IFN- γ The ability of these cells to express CD40 was significantly enhanced under the stimulation of IFN- γ When stimulated, CD40 expression is most significantly upregulated and can stimulate tumor cells to express CD40.