The internalized Ag is processed in intracellular vesicles where it bind MHC class II molecules which then traffic to the cell surface where they can be recognized by T cells. Effector T Cells There are three classes of effector T cell specialized to deal with three classes of pathogens. I molecules at the cell surface. T H 1 and T H 2 cells express CD4 and recognize fragments of antigens degraded within intracellular vesicles and displayed at the cell surface by MHC class II molecules. Proliferating T cells differentiate into armed effector T cells with altered surface properties that do not require co-stimulation to act.
T cells proliferate before they differentiate into effector cells.
A naive T cell that encounters Ag on the surface of an APC receives the required two signals, becomes activated and both secretes and responds to IL-2. Activated CD8 cells can kill any cell infected with a virus, whether or not the infected cell can express co-stimulatory molecules. Armed effector CD4 T cells can activate B cells and macrophages that have taken up antigen, even if, as is often the case, they have too little co-stimulatory activity to activate a naive CD4 T cell. The armed effector T cells cease to recirculate through lymph nodes but instead now express the integrin VLA-4 which allows them to bind to vascular endothelium at sites of inflammation, Naive CD8 T cells are predestined to become cytotoxic cells. Cytotoxic CD8 T cells can induce programmed cell death in target cells.