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Glossary of HIV/AIDS Terms

for Health Care Providers

Glossary of HIV/AIDS Terms

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, and suppressor. They are essential for a normal functioning immune system.

T Lymphocytes

See T cells.

T4 cell

(Also called T-helper cell.) Antibody-triggered immune cells that seek and attack invading microorganisms. Macrophages summon T4 cells to the infection site. There, the T4 cell reproduces and secretes potent chemicals that stimulate B cells to produce 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.


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.

Therapeutic HIV vaccine

Also called treatment vaccine. A vaccine designed to boost the immune response to HIV infection. A therapeutic vaccine is different from a preventive vaccine, which is designed to prevent an infection or disease from becoming established in a person.


Any form of treatment. Drugs, radiation, and psychiatric counseling are forms of therapy.


A decreased number of blood platelets (cells important for blood clotting).

See Platelets; Immune thrombocytopenic purpura (ITP).


Patches in the mouth (sometimes painful) 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.


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. The thymus processes white blood cells (lymphocytes), which kill foreign cells and stimulate other immune cells to produce antibodies.


Three times each day.


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 (Titre)

A laboratory measurement of the amount — or concentration — of a given compound in solution.


Three times each week


Trimethoprim/sulfamethoxazole (generic name); cotrimoxazole, Bactrim, Septra (trade names).

See Cotrimoxazole.


Training of trainers.

Total lymphocyte count

A count of the number of lymphocytes (white blood cells) in the blood.


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

Toxoplasmic encephalitis

See Toxoplasmosis (toxo).

Toxoplasmosis (toxo)

An opportunistic infection caused by the protozoan parasite Toxoplasma gondii, which is found in undercooked meat and cat feces. A common manifestation is toxoplasmic encephalitis, characterized by brain swelling, confusion, lethargy, and possible coma.


A liver enzyme (e.g. AST, ALT). A laboratory test that measures transaminase levels to assess the functioning of the liver.


The process of transfusing fluid (such as blood) into a vein.


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 either during pregnancy or birth, or postnatally through breast-feeding. HIV is now rarely transmitted by transfusion of blood or blood products because of screening measures.


Across or through the placenta. Usually refers to the exchange of nutrients, waste products, and other materials (e.g. drugs) between the developing fetus and the mother. Also refers to transmission of virus such as HIV across the placenta to the infant.

Treatment failure

Inability of a medical therapy to achieve the desired results.

Trimethoprim/sulfamethoxazole (TMP/SMZ, TMP/SMX, cotrimoxazole, Bactrim, Septra)

A combination antibiotic drug effective at preventing and treating Pneumocystis jiroveci pneumonia (PCP); also serves as a prophylaxis against toxoplasmosis.

See Cotrimoxazole.

T-suppressor cells

T lymphocytes responsible for turning off the immune response 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 around the test site, the person is said to be sensitive to tuberculin, and the test is read as positive.

Tuberculosis (TB)

Infection with the bacteria Mycobacterium tuberculosis, as evidenced by a positive tuberculin skin test (TST) that screens for infection with this organism. Sometimes, TST is called a purified protein derivative (PPD) or Mantoux test. A positive skin test might or might not indicate active TB disease. Thus, any person with a positive TST should be screened for active TB and, once active TB is excluded, evaluated for treatment to prevent the development of TB disease. TB infection alone is not considered an opportunistic infection.