CD40, also known as TNFRSF5, is a type I transmembrane glycoprotein and a member of the tumor necrosis factor receptor superfamily. It is primarily expressed on antigen-presenting cells such as B cells, dendritic cells, and monocytes, and plays a pivotal role in the regulation of immune responses. The interaction of CD40 with its ligand, CD40L (CD154), is crucial for T cell activation, B cell proliferation, and the induction of humoral immunity. CD40 signaling is also involved in the maturation of dendritic cells and the promotion of cell-mediated immunity. Additionally, CD40 has been implicated in the pathogenesis of various cancers and is considered a potential target for immunotherapeutic strategies.