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.

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.
 * 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.