Lymphocytes are class of immune cells that specifically recognize and respond to foreign antigens. They are mediators of humoral and cellular immunity & unique cells of adaptive immunity. They can be broadly divided into three major populations on the basis of functional and phenotypic differences: B Cells, T Cells and Natural Killer Cells.
T lymphocytes (T cells) are type of White blood cells which derive their letter designation from their site of maturation i.e. Thymus. T cells expresses a unique antigen-binding receptor called the T-cell receptor. T-cell receptors only recognize processed pieces of antigen (typically peptides) bound to cell membrane proteins called major histocompatibility complex (MHC) molecules.
Immature T cells (termed T-stem cells) migrate to the thymus gland where they mature and differentiate into various types of mature T cells. Those cells which are potentially activated against body’s own tissues are normally killed or changed (“down-regulated”) during maturation process.
T lymphocytes are divided into three cell types T helper (TH) cells, T cytotoxic (TC) cells and T regulatory (TREG) cells. TH and TC cells can be distinguished from one another by the presence of either CD4 or CD8 membrane glycoproteins on their surfaces.
- T cells which display CD4 function as TH cells and recognize antigens in complex with MHC class II.
- T cells which display CD8 function as TC cells and recognize antigen in complex with MHC class I.
Helper T cells mounts immune response by recognizing the presence of a foreign antigen and then stimulating antibody production and produce /secrete cytokines that “turn on” or activate other T cells as well as differentiation of B cells. Regulatory T cells function in an opposite manner i.e. they dampen or turn off immune response. Cytotoxic (Killer) T cells directly attack and destroy cells with antigenic material.
The ratio of CD4+ to CD8+ T cells is approximately 2:1 in normal mouse and human peripheral blood. A change in this ratio is often an indicator of immunodeficiency disease (e.g., HIV), autoimmune diseases, and other disorders.
Naïve CD8+ T cells browse the surfaces of antigen presenting cells with their T-cell receptors. If and when they bind to an MHC-peptide complex, they become activated, proliferate, and differentiate into an effector cell called a cytotoxic T lymphocyte (CTL).
The CTL has a vital function in monitoring the cells of the body and eliminating any cells that display foreign antigen complexed with class I MHC, such as virus-infected cells, tumor cells, and cells of a foreign tissue graft. T-cells hunt down and destroy those cells.
To proliferate and differentiate optimally, naïve CD8+ T cells also need help from mature CD4+ T cells. If and when they recognize an MHC-peptide complex, they can become activated and proliferate and differentiate into one of a variety of effector T cell subsets. T helper type 1 (TH1) cells which regulates immune response to intracellular pathogens, and T helper type 2 (TH2) cells which regulates response to extracellular pathogens.