The natural ligand for the T-cell antigen CD28; (ANTIGENS, CD28); mediating T- and B-cell adhesion. CD80 is expressed on activated B-cells and gamma-interferon-stimulated monocytes. The binding of CD80 to CD28 and $CTLA-$4 provides a co-stimulatory signal to T-cells and leads to greatly upregulated lymphokine production.
