CD80, also known as B7-1, is a 262 amino acid long 60 kDa molecule and a member of the immunoglobulin superfamily. Together with CD86 (B7-2) it belongs to the B7 family of costimulatory molecules. CD80 is expressed on activated B cells, dendritic cells, and monocytes/ macrophages. The interaction of CD80 with its ligands CD28 and CD152 (CTLA-4) plays a critical role in induction and regulation of immune responses. Binding of CD80 to CD28, which is constitutively expressed on T cells, provides a costimulatory signal and induces T cell proliferation and cytokine production. In contrast, binding to CTLA-4 strongly inhibits proliferation and interleukin 2 (IL-2) secretion by T cells.