T cells are lymphocytes that direct and carry out cell-based adaptive immunity. Each one recognizes a single specific antigen displayed on the surface of another cell.
Training in the thymus
T cells are made in the bone marrow and mature in the thymus. Cells that cannot recognize antigen, or that react strongly against the body's own tissues, are removed.
Recognizing antigen
T cells do not see free germs; they see short protein pieces presented on MHC molecules on cell surfaces. Helper T cells read MHC class II on antigen-presenting cells, and cytotoxic T cells read MHC class I, found on nearly every cell.
Helper T cells
Helper T cells release cytokines that activate B cells, macrophages and cytotoxic T cells. HIV infects and destroys them, so a falling CD4 count means weaker immunity.
Cytotoxic T cells
Cytotoxic T cells kill cells infected with viruses and some cancer cells. They also attack transplanted organs, which is why transplant patients take immunosuppressants.
| Cell | Marker | Main job |
|---|---|---|
| Helper T cell | CD4 | Directs the immune response with cytokines |
| Cytotoxic T cell | CD8 | Kills infected and abnormal cells |
| Regulatory T cell | CD4 and CD25 | Calms the response and prevents autoimmunity |
| Memory T cell | CD4 or CD8 | Faster response on re-exposure |
At the bedside
In HIV, a CD4 count below 200 cells per microliter defines AIDS and calls for prophylaxis against opportunistic infections. Patients on steroids, chemotherapy or transplant drugs also have weak T cell defenses, so watch for unusual infections.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).