Thymocytes or T lymphocytes are white blood cells that originate in the bone marrow and develop within the thymus gland. T lymphocytes, also called T cells, are instrumental in the process of cell-mediated immunity, an immune response generated by cellular elements against viruses, tumors, and parasites. T cell activation is the process by which foreign cells turn on a mature, but immunologically naive, T cell and stimulate it to attack the same class of foreign cell through a complex internal sequence of events. This process leads to the development of targeted helper CD4+ T cells or killer and suppressor CD8+ T cells.
First, a large immune cell, called a macrophage, engulfs a tumor cell or virus. The macrophage then displays a protein molecule or antigen of the engulfed material on its surface, becoming an antigen-presenting cell (APC). This conglomerate then binds to a T cell receptor, which leads to T cell activation. CD4+ T cells release chemicals that attract other immune cells, control killer T cells, and stimulate the growth and proliferation of other T cells that are targeted specifically against that antigen. CD8+ cells begin to destroy any body cells that have the targeted antigen, such as tumor cells or infected cells, or they may engage in a suppressor function, shutting down overactive T cells.
Major histocompatibility complexes (MHCs) are genetically coded regions that exist in virtually all vertebrate cells. Proteins coded for by the MHC regions sit on the surface of each cell. These proteins serve as biological markers that identify whether a cell is “self” or “foreign.” Prior to the process of T cell activation, the T cells undergo both positive and negative selection. They must be able to recognize the MHC proteins in order to distinguish the body’s cells from foreign cells, but they must also not bind too strongly to the proteins so that they do not affect the body’s own cells, a condition called autoimmunity.
T cell activation depends on multiple interactions between the APC and molecules on the surface of the T cell. Two signals, provided by the binding interactions, are required for T cell activation. The primary signal occurs when the antigen that is presented by the APC binds to the T cell receptor (TCR). A second signal occurs when a second T cell receptor, called CD28, binds to CD80 or CD86 on the surface of the APC. If the CD28 interaction occurs, the T cell produces chemical messengers, called cytokines, which promote further development and proliferation of the T cell into an immunologically competent cell.