CD28 is a co-stimulatory molecule, a standalone family member, consisting of a single transmembrane domain, primarily located on the surface of T cells. It activates T cells by binding to the B7 family molecules (CD80/CD86), promoting IL-2 production and T cell proliferation. The CD28 signaling pathway is crucial for T cell activation and survival. Its gene expression is highly specific in T cells, and mutations or alterations in expression may affect immune responses and the development of autoimmune diseases.