For antigens to trigger an immune response, they must be recognized by lymphocytes, esp. T
lymphocytes or T cells. T cells arise from stem cells in the bone marrow and are then processed in the
thymus, after which they can differentiate into several different types of T cells. Helper T cells (T H cells)
play a central role in coordination of immune response, releasing cytokines that activate phagocytes
and other lymphocytes. Cytotoxic T cells (TC cells) recognize and destroy body cells that are infected or
cancerous. All T Cells recognize and bind to antigen with a receptor called the T-cell receptor. Different
types of T cells can be distinguished by another surface protein, which has different forms. T H cells have
a surface protein called CD8.
Activation of TH cells by antigen is important to immune response; For TH cells to antigen with their T-cell
receptor, the antigen must be presented to them by antigen-presenting cell (APC), such as a phagocyte,
dendritic cell, or B Cell. When phagocytes or dendritic cells encounter microbes, they take them in
and break them down. As the microbe is being broken down into its component parts, fragments of
microbial antigen are then displayed on the surface of the antigen-presenting cell where they can be
recognized by TH cells.