T-cells are white-blood-cells – or lymphocytes – which mature in the thymus. They protect us against invading cells, or antigens. These antigens cause T-cells to become active and produce antibodies. These antibodies stay attached to the cell that created them and the whole antibody-bearing cell attacks the antigens. T-cells are among the most important cells in the immune system. They originate in the bone marrow and then migrate to the thymus gland (hence the 'T'), where they are trained to respond to a particular target. One T-cell may be subtly altered so that it responds to the polio virus, another to hepatitis-B, a third to the AIDS virus, and so on. Each T-cell has a unique target. When a T-cell leaves the thymus, it goes on the prowl for its enemy. Normally we each have billions of T-cells in our bodies, on the lookout for their individual targets. Once a T-cell spots its enemy, a complex process is set in motion that results in the production of millions of clones. Thus your body can go from having a single scout to well equipped army in a matter of days.
Our bodies produce two main types of T-cells: T-killer cells, which are part of the attack team, and T-helper cells, which direct the whole operation. T-helper cells coordinate other cells in the immune system by producing a family of intercellular signaling substances called 'cytokines'. Each cytokine regulates a different set of cells, suppressing the growth of some and stimulating the growth of others. (Cytokines include the interleukins, interferons, colony-stimulating factors, and tumor necrosis factors.) Without the T-helper cells coordinating the immune system, the battle would be lost. It would be as if you were trying to fight a war when your entire communication network has broken down.
The number of circulating T-cells can give us a clue to the level of an infection. More antigens mean more T-cells.
The low CD4 counts caused by some of these conditions often fall below 200 per cubic millimeter, which is the level needed to diagnose acquired immunodeficiency syndrome (AIDS) in someone who was previously positive for antibodies to the human immunodeficiency virus (HIV-positive).