CD28 is a key costimulatory receptor predominantly expressed on T cells, which plays an essential role in T cell activation, proliferation, and survival. Upon binding to its ligands CD80 (B7-1) and CD86 (B7-2) expressed on antigen-presenting cells, CD28 provides a secondary signal that amplifies the initial T cell receptor engagement with the antigen. This costimulation is crucial for preventing T cell anergy and for the full execution of immune responses, including the production of cytokines and the development of memory T cells. CD28 also contributes to the induction and maintenance of regulatory T cells, and its signaling involves the activation of multiple downstream pathways, including the PI3K-AKT axis, which is important for cell metabolism and survival.