The first step in initiation of the immune response to an antigen must necessarily involve modification of the antigen, and these specialized cells are called APC. Without such processing, T cells cannot recognize antigen. Thus, it is the secretion of cytokines by APC activated by antigen presentation that further activates antigen specific T cells. This interaction between APC and T cells is strongly influenced by a group of molecules called co-stimulators. For example, it is CD80 (B7-1) and CD86 (B7-2) on the APC cells with receptors CD28 and CTLA-4 on the T cell that provides this interaction. The absence of these co-stimulators leads to T-cell unresponsiveness. The importance of this pathway is emphasized by the fact that antagonists to these co-stimulators do interrupt the immune response in both in vitro and in vivo experiments. For example, mice with a severe form of lupus exhibit a milder disease following a CTLA-4 antagonist.