CD80 (B7-1) is a type I transmembrane glycoprotein and a member of the immunoglobulin superfamily of cell surface receptors. It is constitutively expressed by dendritic cells, monocytes, and peritoneal macrophages. Expression of CD80 on B cells is upregulated after activation by various means including stimulation by LPS, IL-4, and surface Ig receptor crosslinking with peak expression occurring between 48 and 72 hours post-activation. The interaction of CD80 with its ligands, CD28 and CD152 (CTLA-4), provides a potent costimulatory signal for T cells activated through the CD3/T cell receptor complex. The monoclonal antibody 1G10 blocks binding of CTLA-4 to CD80 but it does not block stimulation of T cells by natural antigen-presenting cells.