T cells (and B cells) are part of the adaptive immune response that takes 4-7 days after infection to become effective. T cells are lymphocytes or cells of the lymphatic system. T cells are so called because they develop in the thymus, an organ located in the upper chest just above the heart. T cells are of two types: helper T cells (which have a marker on their cell surface called CD4) and killer T cells (which have the CD8 marker on their surface). Killer T cells (also called cytotoxic T lymphocytes or CTLs) directly attack body cells that are infected with a virus or malignant or abnormal tumor cells, cells that antibodies would never perceive.

Helper T cells are the "generals" of the immune system, calling into play and directing the activity of killer T cells. The helper T cells are essential for an effective immune response since they activate other immune cells including most B cells to produce antibody. Individual T cells are targeted against the specific antigen signatures of viruses and bacteria, and when helper T cells encounter their specific antigens, they become activated and quickly expand in number. In AIDS, the virus HIV attacks activated T helper cells, it kills off the very cells that should coordinate the body's antiviral defenses, and as a result people with HIV usually have few or no HIV-specific T helper cells!

How Do T Cells See Their Antigen?

T cells have almost the same diversity as B cells that allow them to recognize various different pathogens. However, T cells recognize a part of the antigen presented on infected cells or cancerous cells or transplanted organ cells (immune rejection). It is absolutely fascinating how T cells know which cells are 'normal' versus 'infected' or 'abnormal' or 'non-self'. T cells have receptors on their surfaces that recognize HLA (human leukocyte antigen) markers that are present on all body cells. These are the markers (along with blood group markers) that are used to determine whether a donor transplanted organ or bone marrow will be accepted or rejected by the recipient. The HLA molecules on the cell surface bind and present protein fragments from within the cell. Thus, they allow the T cells to sample the various proteins within the cell and get activated only if they recognize a 'foreign-looking' protein fragment bound to the HLA molecule on the cell surface.

How Do Helper T Cells 'Help' And Killer T Cells 'Kill'?

T cells act by releasing certain chemicals known as cytokines. Cytokines are proteins (secreted by other immune cells too) that are responsible for activating cells, triggering them to grow and divide or to die. A complex network of cytokines is secreted by the helper T cells that determine the course of the immune response. Killer T cells, on the other hand, release cell-damaging enzymes that create holes in the membranes of the target cells and trigger them to undergo programmed cell death.

Cytokines, are now being used-alone or in combination to treat various disorders such as cancer, rheumatoid arthritis and AIDS among others. The aim of these treatments is to mimic part of the immune response that fools the cells to either get activated and divide such as in the case of the immune deficiency disease AIDS or to slow down a destructive immune response such as in the case of the autoimmune disease rheumatoid arthritis.