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Medical Dictionary


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T (thymine)
In genetics, T stands for thymine, one member of the A-T (adenine-thymine) base pair in DNA. The other base pair in DNA is G-C (guanine-cytosine).

Each base pair forms a "rung of the DNA ladder." A DNA nucleotide is made of a molecule of sugar, a molecule of phosphoric acid, and a molecule called a base. The bases are the "letters" that spell out the genetic code. In DNA, the code letters are A, T, G, and C, which stand for the chemicals adenine, thymine, guanine, and cytosine, respectively. In base pairing, adenine always pairs with thymine, and guanine always pairs with cytosine.

T cell
A type of white blood cell that is of crucial importance to the immune system.

Immature T cells (termed T- stem cells) migrate to the thymus gland in the neck, where they mature and differentiate into various types of mature T cells and become active in the immune system in response to a hormone called thymosin and other factors. T-cells that are potentially activated against the body’s own tissues are normally killed or changed (“down-regulated”) during this maturation process.

There are several different types of mature T cells. Not all of their functions are known.

T cells can produce substances called cytokines, such as the interleukins, which in turn further stimulate the immune response.

T-cell activation is measured as a way to assess the health of patients with the HIV virus (AIDS). T-cell activity is examined less frequently in other disorders, but as we learn more about how the immune system works, it is becoming part of the routine process for assessing many immune conditions and some types of cancer.

T cell are also known as T lymphocytes. The "T" stands for "thymus" -- the organ in which these cells mature. As opposed to B cells which mature in the bone marrow.

T cells, peripheral
T cells found in the peripheral blood rather than in the lymphatic system.

T lymphocytes, cytotoxic
T cells that express the CD8 transmembrane glycoprotein (CD8+ T cells). They are antigen-specific: able to search out and kill specific types of infected cells. When they find cells carrying the viral peptide they are looking for, they induce these cells to secrete proteins that attract nearby macrophages (white blood cells). These macrophages then surround and destroy the infected cells. Cytotoxic T cells are particularly important in the body’s response to viruses and cancer. Abbreviated CTL. Also known as TC cells.

See also T cells and T-supressor cells.

Seen on a prescription, t.i.d. means three times a day. It is an abbreviation for "ter in die" which in Latin means three times a day. The abbreviation t.i.d. is sometimes written without a period either in lower-case letters as "tid" or in capital letters as "TID". However it is written, it is one of a number of hallowed abbreviations of Latin terms that have been traditionally used in prescriptions to specify the frequency with which medicines should be taken. Other examples include:

q.d. (qd or QD) is once a day; q.d. stands for "quaque die" (which means, in Latin, once a day).
b.i.d. (or bid or BID) is two times a day ; b.i.d. stands for "bis in die" (in Latin, twice a day).
q.i.d. (or qid or QID) is four times a day; q.i.d. stands for "quater in die" (in Latin, 4 times a day).
q_h: If a medicine is to be taken every so-many hours, it is written "q_h"; the "q" standing for "quaque" and the "h" indicating the number of hours. So, for example, "2 caps q4h" means "Take 2 capsules every 4 hours."

The symbols T1 through T12 represent the 12 thoracic vertebrae. The thoracic vertebrae are situated between the cervical (neck) vertebrae and the lumbar vertebrae.

The thoracic vertebrae provide attachment for the ribs and make up part of the back of the thorax (the chest).

T-4 cells
T cells that express the CD4 transmembrane glycoprotein (CD4+ T cells). They are active in the body’s immune response, helping to turn this system on when it is challenged by an infection or foreign matter in the body. The HIV virus attacks T-4 cells, knocking out the body’s ability to defend itself against infections. There are at least two types of T-4 cells. Also known as T- helper cells, TH cells.

See also CD4, T cells.

T-4 count
A test that counts the number of T- 4 cells in the blood, usually to assess the immune status of a patient with the HIV virus (AIDS). It’s important to note that there are also T-4 cells in the lymphatic system, and they may be more severely affected there than in the bloodstream. Of the various ways to read this test, the best indicator of health may be the absolute T-4 count. Also known as T-helper count.

See T cells.

T-8 cells
T cells that express the CD8 transmembrane glycoprotein (CD8+ T cells). They close down the immune response after invading organisms are destroyed. Supressor T cells are sensitive to high concentrations of circulating lymphokine hormones, and release their own lymphokines after an immune response has achieved its goal. This signals all other immune-system participants to cease their attack. Some memory B-cells remain after this signal to ward off a repeat attack by the invading organism. Also known as T-supressor cells.

See also CD8, T cells; T lymphocytes, cytotoxic.

An old-fashionned but convenient household measure of capacity. A tablespoon holds about 3 teaspoons, each containing about 5 cc, so a tablespoon = about 15 cc of fluid.