CD40 is a costimulatory protein found on the surface of various cells, including B cells, dendritic cells, and macrophages. It belongs to the tumor necrosis factor receptor (TNFR) superfamily and plays a crucial role in mediating immune responses. The interaction of CD40 with its ligand CD40L is essential for the activation and differentiation of B cells, as well as the enhancement of antigen-presenting cell functions.