The Cluster of differentiation 80 (also CD80 and B7-1) is a B7, type I membrane protein in the immunoglobulin superfamily, with an extracellular immunoglobulin constant-like domain and a variable-like domain required for receptor binding. It is closely related to CD86, another B7 protein (B7-2), and often works in tandem. Both CD80 and CD86 interact with costimulatory receptors CD28 and CTLA-4 (CD152). CD80 can be found on the surface of various immune cells, including B-cells, monocytes, or T-cells, but most typically at antigen-presenting cells (APCs) such as dendritic cells. CD80 has a crucial role in modulatating T-cell immune function as a checkpoint protein at the immunological synapse. CD80 is the ligand for the proteins CD28 (for autoregulation and intercellular association) and CTLA-4 (for attenuation of regulation and cellular disassociation) found on the surface of T-cells. Another ligand of CD80 is programmed death-ligand 1 (PD‐L1), expressed on the surface of T-cells, B-cells, DCs, and macrophages. This interaction is inhibiting and causes a reduction in T-cell activation as well as reduction of cytokine production. Its dissociation constant with CD80 is between the CD28 and CTLA-40 (Kd = 1.4 μM).