Cluster of Differentiation 80, also called B7-1, is a member of cell surface immunoglobulin superfamily which plays key, yet distinct roles in the activation of T cells. It is the ligand for two different proteins on the T cell surface: CD28 and CTLA-4. Studies have shown that CTLA-4 binds mostly to CD80. The structure presents two extracellular domains: a membrane distal variable-like domain (IgV) and a membrane proximal Ig constant-like domain (IgC) along with an intracellular domain. Both IgV and IgC consist of anti-parallel beta sandwiches joined by a short linker region. CD80 is mostly expressed on the surface of antigen-presenting cells including activated B cells, macrophages and dendritic cells.