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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 (T_CM). The T_CM cells are thought to represent memory stem cells. T_CM 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 (T_EM)
  • effector memory RA (T_EMRA)

Memory T cells can be recognized by the differential expression of certain molecules.

• Central memory T_CM cells express L-selectin and the chemokine receptor CCR7, they secrete IL-2, but not IFNγ or IL-4.

• Effector memory T_EM 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 T_CM and T_EM 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

1. ^ Willinger T, Freeman T, Hasegawa H, McMichael A, Callan M (2005). "Molecular signatures distinguish human central memory from effector memory CD8 T cell subsets". J Immunol 175 (9): 5895-903. PMID 16237082. 

Further reading

• Janeway CA, Jr. et al (2005). Immunobiology. (6th ed. ed.), Garland Science. ISBN 978-0-443-07310-6. 

• Cellular and Molecular Immunology (5th Ed.) Abbas AK, and Lichtman, Editor: Saunders, Philadelphia, 2003.

Wikipedia content modification information:

• This page was last modified on 12 May 2008, at 22:00.

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