CD80 (also known as B7-1) is a type I membrane protein that is a member of the immunoglobulin superfamily, with an extracellular immunoglobulin constant-like domain and a variable-like domain required for receptor binding. It is expressed on antigen-presenting cells (APCs), including B cells, dendritic cells, monocytes, and macrophages. CD80 is the receptor for the proteins CD28 and CTLA-4 found on the surface of T-cells. It is involved in the costimulatory signal essential for T-lymphocyte activation. T-cell proliferation and cytokine production is induced by the binding of CD28, binding to CTLA-4 has opposite effects and inhibits T-cell activation. CD80 also acts as a cellular attachment receptor for adenovirus subgroup B. (PMID: 7545666; 12015893; 16920215)