There are many types of T cells:
Every effective immune response involves T cell activation, however they are especially important in cell-mediated immunity, that is the defense against pathogenic organisms inside body cells, tumor cells and is also involved in rejection reactions.
CD4 and CD8 refer to the characteristic antigens on the surface of the different sub-types of T-lymphocytes. These CD molecules are convenient diagnostic markers for identifying and quantitating these cells by flow cytometry using specific antibodies against them. T cell activity and secretions are frequently determined using the ELISPOT technique.
Throughout life, a source of all lymphocytes remains bone marrow. Progeny of multipotential lymphoid stem cells (CFU-L) that are destined to become T cells, move into thymus, where they are called thymocytes.
Thymocytes are immature T cells. The differentiation called thymic education occurs in a few stages in the subcapsular region of the thymic cortex. At first, multipotential T cells move at the edge of the thymic cortex. These cells express CD2,CD7 and CD3, but neither CD4 nor CD8 (they are double negative, CD4-CD8- cells). In the early stage of differentiation, they start to express CD2 and CD7 molecules. This is followed by expression of the CD1 molecule, that marks the midstage of T cell differentiation. At the end of differentiation they express TCRs, CD3, CD4 and CD8 molecules. Because they have CD4 and CD8 molecules at their surface, they are said to be double positive (CD4+CD8+).
Double positive cells move deeper in the cortex, where they get presented with self and foreign antigens in a complex with type I and type II MHC molecules by type II and type III cortical epithelioreticular cells. Only those cells that recognize the antigen presented and bind the MHC molecule with the affinity high enough, survive. Other cells go into apoptosis and their remains are engulfed by macrophages. This process is called positive selection.
All their life T cells recognize only those antigens, for which they are specific and that are binded in a complex with the MHC molecule of the type they were presented with in the thymic cortex. They are said to be limited with type I or type II MHC molecules.
The cells that survive positive selection move towards the thymic medulla where at the boundary between thymic cortex and thymic medulla and in the medulla, they get are presented with self-antigens in a complex with MHC molecules on the surface of type V and VI thymic epithelioreticular cells. If they do recognize these antigens, they die by apoptosis. Only those cells survive that do not recognize them. Other T cells die by apoptosis (a small fraction actually survives but normally is controlled so it does not cause autoimmune diseases). This process is called negative selection.
T cells go into apoptosis if they cannot express their TCR receptors, if they aren't positively selected or if they get removed by negative selection. About 98% of T cells die in the thymus. Their remains get engulfed by macrophages. Only 2% of T cells mature and are selected to survive. These cells lose either their CD4 molecules or CD8 molecules and leave the thymus via postcapillary venules.
Hormonal substances (thymosin, interferon γ, interleukins, colony stimulating factors, thymopoetin) secreted by type VI epithelioreticular cells within the thymic (Hassal's) corpuscles promote the process of thymic cell education.
It is important to note that not only is the differentiation of T cells controlled by thymic epithelioreticular cells, but also T cells influence the microarchitecture of thymic epithelioreticular cells. The interaction is bidirectional. This phenomenon is called "cross-talk".