The activation of T cells begins when they encounter an antigen-presenting cell (APC), such as a dendritic cell or macrophage, displaying a foreign antigen. The T cell receptor (TCR) on the T cell surface binds to the antigen-MHC (Major Histocompatibility Complex) complex on the APC, leading to T cell activation. This process involves multiple signals: the primary signal via the TCR and co-stimulatory signals provided by molecules such as CD28.
