T

3TC:
See Lamivudine.

TAT:
(tat) One of the regulatory genes of HIV. Three HIV regulatory genes -- tat, rev, and nef -- and three so-called auxiliary genes -- vif, vpr, and vpu -- contain information necessary for the production of proteins that control the virus ability to infect a cell, produce new copies of the virus, or cause disease. The tat gene is thought to enhance virus replication.

TB:
See Tuberculosis.

T CELLS:
(T Lymphocytes.) T cells are white blood cells, derived from the thymus gland, that participate in a variety of cell-mediated immune reactions. Three fundamentally different types of T cells are recognized: helper, killer (see killer T cells), and suppressor. They are the immune system's "border police," responsible for finding infected or cancerous cells. The killer T cell receptors (TCR) bind to an infected cell's distress signal -- a combination of one of the cell's own proteins and a tiny fragment of the invader's protein. The bits of foreign protein are made with the help of enzymes inside the invaded cell that chew up the pathogens into protein fragments (peptides), which are then scooped up by the major histocompatibility complex (see MHC) and carted through the cell membrane.

T4 CELL:
(Also called T-helper cell). Antibody-triggered immune cells that seek and attack invading organisms. Macrophages summon T4 cells to the infection site. There the T4 cell reproduces and secretes its potent lymphokines that stimulate B cell production of antibodies; signal natural killer or cytotoxic (cell-killing) T cells; and summon other macrophages to the infection site. In healthy immune systems, T4 cells are twice as common as T8 cells. If a person has AIDS, the proportion is often reversed. The virus enters T4 cells through its receptor protein and encodes its genetic information into the host cell's DNA, making T cells virtual viral factories. HIV-infected T4 cells may not die, but, rather, may cease to function. They also begin to secrete a substance known as Soluble Suppressor Factor that inhibits the functioning of even unaffected T cells.

T8 CELL:
(Also called killer cells; see Cytotoxic T-lymphocyte). Immune cells that shut down the immune response after it has effectively wiped out invading organisms. Sensitive to high concentrations of circulating lymphokines, T8 cells release their own lymphokines when an immune response has achieved its goal, signaling all other participants to cease their coordinated attack. A number of B lymphocytes remain in circulation in order to fend off a possible repeat attack by the invading organism. With HIV, however, the immune system's response system does not work. T4 cells are dysfunctional, lymphokines proliferate in the bloodstream, and T8 cells compound the problem by misreading the oversupply of lymphokines as meaning that the immune system has effectively eliminated the invader. So while HIV is multiplying, T8 cells are simultaneously attempting to further shut down the immune system. The stage is set for normally repressed infectious agents, such as PCP or CMV, to proliferate unhindered and to cause disease.

TEMPLATE:
A gauge, pattern, or mold used as a guide to the form of the piece being made. In biology, a molecule -- such as DNA -- that serves as a pattern for the generation of another macromolecule (e.g., messenger RNA). See Ribonucleic Acid.

TERATOGENICITY:
The production of physical defects in offspring in utero (i.e., causing birth defects). Teratogenicity is a potential side effect of some drugs, such as thalidomide.

TERRY BEIRN COMMUNITY PROGRAMS FOR CLINICAL RESEARCH ON AIDS:
See Community Programs for Clinical Research on AIDS.

TESTOSTERONE:
Naturally occurring male hormone. When administered as a drug it can cause gain in lean body mass, increased sex drive, and possibly aggressive behavior. Many men with HIV have low testosterone levels.

THERAPEUTIC HIV VACCINE:
Also called treatment vaccine. A vaccine designed to boost the immune response to HIV in persons already infected with the virus. A therapeutic vaccine is different from a preventive vaccine, which is designed to prevent a disease from becoming established in a person.

Th1 RESPONSE:
An acquired immune response whose most prominent feature is high cytotoxic T lymphocyte activity relative to the amount of antibody production. The Th1 response is promoted by CD4+ "Th1" T-helper cells. See Th2 Response.

Th2 RESPONSE:
An acquired immune response whose most prominent feature is high antibody production relative to the amount of cytotoxic T lymphocyte activity. The Th2 response is promoted by CD4+ "Th2" T-helper cells. See Th1 Response.

THROMBOCYTOPENIA:
A decreased number of blood platelets (cells important for blood clotting). See Platelets; Immune Thrombocytopenia Purpura.

THRUSH:
Sore patches in the mouth caused by the fungus Candida albicans. Thrush is one of the most frequent early symptoms or signs of an immune disorder. The fungus commonly lives in the mouth, but only causes problems when the body's resistance is reduced either by antibiotics that have reduced the number of competitive organisms in the mouth, or by an immune deficiency such as HIV disease. See Candidiasis.

THYMOSIN:
A polypeptide hormone of the thymus gland that influences the maturation of T cells destined for an active role in cell-mediated immunity.

THYMUS:
A mass of glandular tissue (lymphoid organ) found in the upper chest under the breastbone in humans. The thymus is essential to the development of the body's system of immunity beginning in fetal life (i.e., before birth). The thymus processes white blood cells (lymphocytes), which kill foreign cells and stimulate other immune cells to produce antibodies. An important function of the thymus is to weed out lymphocytes that react to proteins produced by the body (self-antigens), thus preventing autoimmune disease. The gland grows throughout childhood until puberty and then gradually decreases in size. See Thymosin.

TISSUE:
A collection of similar cells acting together to perform a particular function. There are four basic tissues in the body: epithelial, connective, muscle, and nerve.

TITER:
(Also titre.) A laboratory measurement of the amount -- or concentration -- of a given compound in solution.

T LYMPHOCYTE PROLIFERATION ASSAY:
Measures the strength of response of T memory cells -- a subgroup of T lymphocytes -- to HIV.

T LYMPHOCYTES:
See T Cells.

TOXICITY:
The extent, quality, or degree of being poisonous or harmful to the body.

TOXOPLASMIC ENCEPHALITIS:
See Toxoplasmosis.

TOXOPLASMOSIS:
Toxoplasmosis is an infection that is caused by the protozoan (see protozoa) parasite, Toxoplasma gondii. The parasite is carried by cats, birds, and other animals, and is found in soil contaminated by cat feces and in meat, particularly pork. The parasite can infect the lungs, retina of the eye, heart, pancreas, liver, colon, and testes. Once T. gondii invades the body, it remains there, but the immune system in a healthy person usually prevents the parasite from causing disease. If the immune system becomes severely damaged, as in HIV-infected persons, or is suppressed by drugs, T. gondii can begin to multiply and cause severe disease. In HIV-infected persons, the most common site of toxoplasmosis is the brain. When T. gondii invades the brain, causing inflammation, the condition is called toxoplasmic encephalitis. While the disease in HIV-infected persons can generally be treated with some success, lifelong therapy is required to prevent its reoccurrence.

TRANSAMINASE:
A liver enzyme. A laboratory test that measures transaminase levels is used to assess the health of the liver.

TRANSCRIPTION:
The process of constructing a messenger RNA molecule, using a DNA molecule as a template, with the resulting transfer of genetic information to the messenger RNA. As related to HIV: The process by which the provirus produces new viruses. RNA copies, called messenger RNA, must be made that can be read by the host cell's protein-making machinery. Cellular enzymes, including RNA polymerase II, facilitate transcription. The viral genes may partly control this process. For example, tat encodes a protein that accelerates the transcription process by binding to a section of the newly made viral RNA. See Integration; Ribonucleic Acid.

TRANSFER FACTOR:
A fraction of white blood cells that apparently "transfers" capability to mount an immune response to a specific antigen.

TRANSFUSION:
1. The process of transfusing fluid (such as blood) into a vein. 2. The transfer of whole blood or blood products from one individual to another.

TRANSLATION:
As related to HIV: The process by which HIV messenger RNA is processed in a cell's nucleus and transported to the cytoplasm, the cellular material outside the nucleus. In the cytoplasm, the cell's protein-making machinery translates the messenger RNA into viral protein and enzymes.

TRANSMISSION:
In the context of HIV disease: HIV is spread most commonly by sexual contact with an infected partner. The virus can enter the body through the mucosal lining of the vagina, vulva, penis, rectum, or, rarely, the mouth during sex. The likelihood of transmission is increased by factors that may damage these linings, especially other sexually transmitted diseases that cause ulcers or inflammation. HIV also is spread through contact with infected blood, most often by the sharing of drug needles or syringes contaminated with minute quantities of blood containing the virus. Children can contract HIV from their infected mothers during either pregnancy or birth, or postnatally, through breast-feeding. In developed countries, HIV is now only rarely transmitted by transfusion of blood or blood products because of screening measures.

TREATMENT IND:
A program to provide experimental treatments to a class of patients who lack satisfactory alternative treatments. IND stands for Investigational New Drug application, which is part of the process to get approval from the FDA for marketing a new prescription drug in the U.S.

TRIGLYCERIDE:
A compound made up of a fatty acid (such as oleic, palmitic, or stearic acid) and glycerol. Triglycerides make up most animal and vegetable fats and are the basic water-insoluble substances (lipids) that appear in the blood where they circulate. In the blood they are bound to proteins, forming high- and low-density lipopro-teins. Elevations of triglyceride levels (particularly in association with elevated cholesterol) have been correlated with the development of atherosclerosis, the underlying cause of some heart diseases and stroke. In relation to HIV disease, there are some patients receiving combination therapies who have experienced significant elevation in their triglyceride levels.

T SUPPRESSOR CELLS:
T lymphocytes responsible for turning the immune response off after an infection is cleared. They are a subset of the CD8+ lymphocytes.

TUBERCULIN SKIN TEST (TST):
A purified protein derivative (PPD) of the tubercle bacilli, called tuberculin, is introduced into the skin by scratch, puncture, or intradermal injection. If a raised, red, or hard zone forms surrounding the test site, the person is said to be sensitive to tuberculin, and the test is read as positive.

TUBERCULOSIS (TB):
A bacterial infection caused by Mycobacterium tuberculosis. TB bacteria are spread by airborne droplets expelled from the lungs when a person with active TB coughs, sneezes, or speaks. Exposure to these droplets can lead to infection in the air sacs of the lungs. The immune defenses of healthy people usually prevent TB infection from spreading beyond a very small area of the lungs. If the body's immune system is impaired because of infection with HIV, aging, malnutrition, or other factors, the TB bacterium may begin to spread more widely in the lungs or to other tissues. TB is seen with increasing frequency among persons infected with HIV. Most cases of TB occur in the lungs (pulmonary TB). However, the disease may also occur in the larynx, lymph nodes, brain, kidneys, or bones (extrapulmonary TB). Extrapulmonary TB infections are more common among persons living with HIV. See Multidrug Resistant TB.

TUMOR NECROSIS FACTOR (TNF):
A cytokine, produced by macrophages, which helps activate T cells. It also may stimulate HIV activity. TNF levels are very high in persons with HIV, and the molecule is suspected to play a part in HIV-related wasting, neuropathy, and dementia. TNF triggers a biochemical pathway that leads to the programmed form of cell suicide known as apoptosis. It also activates a key molecule that can block this very pathway, and so set up a delicate life-death balance within the cell.