T lymphocyte
Meaning of T lymphocyte: White cells schooled in a gland behind the breastbone, and the reason a transplanted organ is recognized as belonging to somebody else.
Definition of T lymphocyte
A T lymphocyte is a white blood cell that recognizes fragments of protein displayed on the surface of other cells and reacts to anything it reads as foreign. Each one carries a receptor unique to itself and its descendants. Some kill infected or abnormal cells directly, some direct the rest of the immune response, and some hold that response in check.
What does the T stand for?
The thymus, a gland behind the breastbone where these cells finish their training. All of them start in the bone marrow, then travel to the thymus to be tested. Cells whose receptor cannot read the body's own display system are discarded, and so are cells that react too strongly to the body's own proteins. Only a small fraction survives both tests and reaches the circulation. A cell still inside the thymus working through that process is a thymocyte, an immature stage rather than another word for the finished cell.
How does it differ from a B lymphocyte?
| Feature | T lymphocyte | B lymphocyte |
|---|---|---|
| Trained in | Thymus | Bone marrow |
| Recognizes | Fragments displayed by another cell | Whole molecules floating free |
| Main weapon | Direct killing and chemical signals | Antibody |
| Depends on | Cells that display fragments | Helper T cells, for most responses |
The two work as a pair rather than as alternatives. A B lymphocyte usually cannot mount a strong or lasting antibody response without a helper T cell approving it, which is why a shortage of T cells weakens antibody defense as well.
What are the main types?
Helper cells carry the CD4 marker and coordinate everything else, releasing signals that tell other cells what kind of response to mount, and they are the population destroyed in untreated HIV infection. Killer cells carry CD8, inspect the fragments a cell displays from its own interior, and destroy any cell showing viral or abnormal protein, which is the work of the cytotoxic T lymphocyte. Regulatory cells restrain the others and prevent attacks on healthy tissue. Memory cells persist for years and react faster on a second encounter, which is what vaccination exploits.
Also known as
Used in a sentence
Flow cytometry showed a marked reduction in the circulating T lymphocyte count.
Good to know
Thymocyte names an immature T cell still developing inside the thymus rather than a synonym for the mature cell found in blood, and the CD4 helper population is the one lost in untreated HIV infection.
Word origin
Where “T lymphocyte” comes from
Lymphocyte joins the Latin lympha, meaning clear water, with the Greek kytos, a hollow vessel and so a cell; the T marks the thymus, where these cells mature.