Another function of T cells is to stimulate helper T cells (also known as CD4 cells), which are types of white blood cells that release cytokines. Cytokines are proteins that help other white blood cells communicate with each other. Because of the important role helper T cells play in communication, they can be thought of as the generals of the immune system. While most of the helper T cells die after the foreign substance has cleared the body, some remain in the body in a resting state. Those remaining helper T cells are known as memory cells. By remaining in the body, memory cells can easily recognize the same foreign substance in the future since they have already been sensitized to them.
Another very important type of T cells is the killer T cell (also known as a cytotoxic T cell or CD8 cell). Killer T cells are like the soldiers of the immune system since they directly attack foreign cells and destroy them. The process of white blood cells directly defending the body by destroying assumed foreign substances is known as cell-mediated immunity.
A T cell (pictued below) is a type of white blood cell that directs the body s immune (defense) system to defend against bacteria and other harmful cells. White blood cells are cells that help protect the body against disease by fighting infectious organisms. T cells have receptors that allow them to recognize foreign substances. There are many different types of T cells.
WHAT ARE SOME MORE SPECIFIC FUNCIONS OF T CELLS?
When activated by substances considered to be foreign (e.g., bacteria), T cells increase in number and perform various functions to help defend the body. One function is to stimulate and suppress the functions of B cells. B cells are types of white blood cells that have the ability to recognize antigens, which are substances located on the surface of invading microorganisms (e.g., bacteria). T cells also suppress certain activities of the immune system so that it does not mistakenly view normal parts of the body as foreign. T cells that perform this function are known as suppressor T cells or regulatory T cells.
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HOW ARE T CELLS MEASURED?
Blood cell tests can be done to count the number of T cells in the body. T cells can also be examined under a very powerful microscope (known as an electron microscope) and flow cytometry. Flow cytometry is a technique for counting, sorting, and separating microscopic particles that are suspended in a stream of fluid.
Such tests can be helpful when attempting to diagnose conditions characterized by a deficient immune system. An example of such a condition is AIDS (Acquired Immunodeficiency Syndrome). AIDS is a decrease in the effectiveness of the body's immune (defense) system due to infection from a virus known as HIV (human immunodeficiency virus). In AIDS, the number of T cells is abnormally decreased. In other conditions, such as infections and blood diseases, there is an abnormal increase in T cells.
T cell levels can also be helpful when trying to diagnose diseases of the lymphatic system. The lymphatic system is a system of vessels that drain lymph from all over the body back into the blood. Lymph is a milky fluid that contains proteins, fats, and white blood cells (which help the body fight off diseases).