Memory T cells
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.
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. 
- 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.
- 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.