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, CTLA-4 (CD152) and the p75 neurotrophin receptor. 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 modulating T-cell immune function as a checkpoint protein at the immunological synapse.