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A lymphocyte is shown in the center of this picture
1. After the naive T cell (N) encounters an antigen it becomes activated and begins to proliferate (divide) into many clones or daughter cells.
2. Some of the T cell clones will differentiate into effector T cells (E) that will perform the function of that cell (e.g. produce cytokines in the case of helper T cells or invoke cell killing in the case of cytotoxic T cells).
3. Some of the cells will form memory T cells (M) that will survive in an inactive state in the host for a long period of time until they re-encounter the same antigen and reactivate.
Memory T cells are a specific type of infection-fighting T cell (also known as a T lymphocyte) that can recognize foreign invaders such as bacteria or viruses, that were encountered during a prior infection or vaccination. At a second encounter with the invader, memory T cells can reproduce to mount a faster and stronger immune response than the first time the immune system responded to the invader. This behaviour is utilized in T lymphocyte proliferation assays, which can reveal exposure to specific antigens.
Sub-populations[]
Within the human cytotoxic T cell population, three distinct sub-populations have now been described:
- central memory (TCM). The TCM cells are thought to represent memory stem cells. TCM display a capacity for self-renewal due to high levels of phosphorylation of an important transcription factor known as STAT5. [1]
- two highly related effector memory sub-types, which strongly express genes for molecules essential to the cytotoxic function of CD8 T cells:
- effector memory (TEM)
- effector memory RA (TEMRA)
Memory T cells can be recognized by the differential expression of certain molecules.
- Central memory TCM cells express L-selectin and the chemokine receptor CCR7, they secrete IL-2, but not IFNγ or IL-4.
- Effector memory TEM cells, however, do not express L-selectin or CCR7 but produce effector cytokines like IFNγ and IL-4.
Antigen-specific memory T cells against viruses or other microbial molecules can be found in both TCM and TEM subsets. Although most information is currently based on observations in the Cytotoxic T cells (CD8-positive) subset, similar populations appear to exist for both the Helper T cells (CD4-positive) and the cytotoxic T cells.
See also[]
- Naive T cell
- V(D)J recombination of DNA
- Memory B cell
References[]
Further reading[]
- Janeway CA, Jr. et al (2005). Immunobiology., 6th ed., Garland Science.
- Cellular and Molecular Immunology (5th Ed.) Abbas AK, and Lichtman, Editor: Saunders, Philadelphia, 2003.
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