CD40, also known as TNFRSF5 or Bp50, is a 48 kDa type I membrane glycoprotein belonging to the tumor necrosis factor receptor superfamily. It is widely expressed on various immune cells, including B cells, dendritic cells, and macrophages. The interaction between CD40 and its ligand, CD40L, is crucial for immune cell activation and regulation. Structurally, CD40 consists of a 62 amino acid cytoplasmic tail, a 22 amino acid transmembrane domain, and a 173 amino acid extracellular domain. This co-stimulatory protein plays a key role in B cell activation, proliferation, and differentiation, and is also involved in immune responses against infections and tumors. Due to its significant role in the immune system, CD40 has become an important therapeutic target, with various agonistic and antagonistic antibodies being developed to modulate its activity.