TAT: One of the regulatory genes of the HIV virus. 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's ability to infect a cell, produce new copies of the virus or cause disease. The tat gene is thought to enhance virus replication. See also nef; rev.

TB: See Tuberculosis.

T CELLS: (T Lymphocytes). A thymus-derived white blood cell that participates in a variety of cell-mediated immune reactions. Three fundamentally different types of T cells are recognized: helper, killer and suppressor (each has many subdivisions). T lymphocytes are CD3+ and can be separated into the CD4+ T helper cells and the CD8+ cytotoxic/suppresssor cells. See also Thymus.

THERAPEUTIC HIV VACCINE: A vaccine designed to boost the immune response to HIV in persons already infected with the virus.

3TC: Also known as Lamivudine, 3TC is composed of the (-) enantiomer of the racemic mixture 2'-deoxy-3'-thiacytidine. Like other nucleoside analogs, 3TC inhibits HIV replication through viral DNA chain termination. It has been used in clinical trials in combination with AZT. See also AZT; Nucleoside Analog.

THRUSH: Sore patches in the mouth caused by the fungus Candida albicans. Thrush is one of the most frequent early symptoms 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 also Candida.

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.


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

TOXOPLASMOSIS: Toxoplasmosis is an infection that is caused by the protozoan 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 people with AIDS, or is suppressed by drugs, T. gondii can begin to multiply and cause severe disease. 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 can occur in persons with healthy immune systems, it can normally be cured successfully. See also Protozoa.

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. Transcription is facilitated by cellular enzymes, including RNA polymerase II. The viral genes may partly control this process: tat, for example, encodes a protein that accelerates the transcription process by binding to a section of the newly made viral RNA. See also Integration; Messenger RNA; tat; Template.

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. See also Cytoplasm; Enzyme; Messenger RNA; Nucleus.

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, very 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. Studies of SIV infection of the genital membranes of nonhuman primates suggest that the sentinel cells known as mucosal dendritic cells may be the first cells infected. Infected dendritic cells may migrate to lymph nodes and infect other cells. 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 either during pregnancy or birth, or postnatally, via breastfeeding. Current research indicates that the AIDS virus may be 100 to 1000 times more contagious during the first two months of infection, when routine AIDS tests are unable to tell whether people are infected. See also Lymph Nodes; Simian Immunodeficiency Virus.

TREATMENT IND: A program to provide experimental treatments to a class of patients who lack satisfactory alternative treatment. IND stands for Investigational New Drug application, which is part of the process to get approval from the Food and Drug Administration for marketing a new prescription drug in the US. See also Investigational New Drug.

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. Repeated 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 to other tissue.