CD28 and Autoimmunity: The second signal is the “costimulatory signal” delivered bycostimulatory molecules expressed on antigen-presenting cells to receptors expressedon T cells.32 The best studied costimulatory signals are those that include CD28/CD80/B7-1,which contribute to the function of the T effector cells and the CTLA-4/CD86/B7-2,which play a decisive role in maintaining peripheral tolerance and impeding autoimmunity.33 On the other hand, the programed cell death 1 receptor (PD-1) has beenidentified as another inhibitory receptor that is expressed on the surface ofactivated T cells.