T cells are lymphoid cells that are created in the bone marrow and then migrate to the thymus gland to mature into a lymphocyte that circulates between the blood and lymph to help aid in the immune response.
Mature T cells are antigen specific, which means that they only bind with one kind of antigen each. Each is identified by surface proteins called clusters of differentiation, or CDs.
All T cells have the CD3 marker, but there are other subsets.
There are six kinds of T cells.
Cytotoxic T cells are CD8+ T lymphocytes that destroy microorganisms through the release of perforin, which creates lesions in the outer membranes of microorganisms, and proteolytic enzymes, which break down proteins. They protect the body from viruses, tissue rejection and new malignant cells.
Read as 'gamma delta', these T lymphocytes make up a very small percentage of T cells and are found mostly in the digestive tract. The antigenic molecules that stimulate gamma delta T cells are unknown.
Helper T cells are CD4 T lymphocytes that aid in cell- and antibody-mediated immune responses. They release chemical messengers that stimulate B-cells and other T cells to bind with antigens.
Memory T cells are those derived from either B or T lymphocytes that have the ability to recognise a foreign antigen to which the body has been previously exposed. They stimulate helper and cytotoxic T cells and can retain the memory of previous infections for years, which provides a durable immune response.
Natural killer cells are large, granular lymphocytes that bind to cells and kills them with cytotoxins. They do not have surface markers and can be activated without previous exposure. They destroy cells that are infected with viruses as well as some kinds of tumour cells.
Formerly known as suppressive T cells, these are a type of lymphocyte that inhibits CD4+ and B cell activity. They appear toward the end of an immune response to stop the actions of other T cells.