T cell activation requires two signals. The first signal is the engagement of the T Cell Receptor (TCR) with the antigen-MHC complex on the surface of APCs. The second signal, known as co-stimulation, involves interactions between co-stimulatory molecules on the T cell and the APC. CD28 on T cells binding to B7 molecules on APCs is a well-studied co-stimulatory interaction. Upon receiving both signals, T cells undergo clonal expansion and differentiate into effector cells capable of targeting and eliminating pathogens.
