CD28 is a type I disulfide-linked homodimer that is constitutively expressed on most thymocytes, at low density on nearly all CD4+ and CD8+ peripheral T lymphocytes, and at very low levels on NK cells. Its expression is upregulated upon T-cell activation. 1-4 CD28 is a ligand for CD80/B7-1 and CD86/B7-2 on B cells and other antigen presenting cells, and plays an important role in the interaction between T cells and B cells. CD28 is a co-stimulatory receptor involved in many, but not all, T-cell independent immune responses.5-8