When a naïve T cell is activated it undergoes clonal expansion and differentiation. 

Antigen-presenting cells (APCs) are a critical component in naïve T cell activation. APCs circulate the body collecting pieces of foreign cells in the form of antigens and carry them back to the secondary lymphoid organs.

On encountering a T cell expressing the matching TCR (T cell receptor), the antigen-MHC complex binds to the TCR, activating it and triggering a signaling cascade.  However, this single interaction isn't enough to activate most naïve cells. Three binding signals must take place simultaneously to achieve full activation and proliferation of naïve T cells - TCR binding, cytokine signaling, and the binding of a co-stimulatory molecule such as CD28.

Once T cells are fully activated by the APC, they differentiate into effector cells and begin the process of eliminating the foreign intruder.