A T cell is a type of lymphocyte or white blood cell which is involved in the function of the immune system. T cells can bind to various cells in the body to kill off infected cells and attack antigens which could cause someone to get sick. A decline in the level of T cells can indicate that someone is suffering from a disease which causes immune suppression, or that someone is taking medications which suppress the functions of the immune system, as is done to prepare for organ transplants.
These cells originate in the bone marrow, working their way into a gland called the thymus, where they mature: the "T" in "T cell" comes from "thymus." In the thymus, the cells differentiate into different types of T cells such as helper cells, natural killer cells, regulatory cells, and cytotoxic cells. These cells in turn travel through the bloodstream to look for signs of unwanted invaders, at which point the cells swing into action to neutralize or destroy the invaders they have identified.
Another important event occurs in the thymus when T cells mature. The thymus goes through a process known as “negative selection,” in which T cells which could trigger autoimmune responses are allowed to die off. This is designed to prevent the immune system from attacking the body by accident. Sometimes this process goes awry, causing autoimmune conditions which can cause severe medical problems.
A T cell can do all sorts of things, depending on which type it is. Some bind to cells and kill them if the cells have become infected, while others store the memory of specific antigens so that the body can respond quickly if these antigens are identified. Helper T cells identify situations in which an immune system response is needed, and trigger the release of various signals to the rest of the body. Regulatory T cells mop up in the aftermath of an infection, taking care to remove cells which have developed autoimmune responses.
In individuals with suspected immune conditions, a doctor may request a test which is designed to count the T cells and to identify numbers of particular types. CD4 cells, for example, are a kind of T cell which decline radically in patients with HIV. In people of normal health, the CD4 count is usually above 1,000, while AIDS patients have 200 or fewer CD4 cells in their blood tests. If a test reveals that a patient has an unusually low T cell count, the doctor needs to determine why, and formulate a plan of action for addressing the issue.