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 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 infected cell that break down the pathogens into protein fragments (peptides), which are then picked up by the major histocompatibility.

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

(Also called T-helper cell.) Antibody-triggered immune cells that seek and attack invading micro-organisms. Macrophages summon T4 cells to the infection site. There the T4 cell reproduces and secretes its potent lymphokines 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. If a person has AIDS, the proportion is often reversed. The virus enters a T4 cell 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 unaffected T cells.

White blood cells with the CD8 protein on their surface. These white blood cells kill some cancer cells and cells infected by intracellular pathogens (some bacteria, viruses, and mycoplasma). Also called cytotoxic T cells, T8 cells, cytotoxic T lymphocytes.

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.

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.

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

When T. gondii (a protozoan parasite) 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.

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 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. While the disease in HIV-infected persons can generally be treated with some success, lifelong therapy is required to prevent its reoccurrence.

A blood test for syphilis.

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

The process by which HIV messenger RNA is processed in a cell's nucleus and transported to the cytoplasm, the component of the cell outside of the nucleus. In the cytoplasm, the cell's protein-making machinery translates the messenger RNA into viral proteins and enzymes.

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 infections 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 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 foetus and the mother.

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 defences 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 HIV infection, 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 HIV-infected persons. 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.