Basic components of the immune system part 41

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.