Page 427 - Veterinary Immunology, 10th Edition

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tend to accumulate under epithelial surfaces, while CD4 memory
VetBooks.ir cells are scattered through the tissues in memory lymphocyte
clusters. These cells slowly divide and replenish their numbers. In
fact, they can be thought of as adult stem cells. IL-7 and IL-15 are
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required for the survival of memory CD8 T cells, whereas only IL-7
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is needed for the survival of CD4 T cells. These maintain the cells
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in a state of slow proliferation. In humans, memory CD4 T cells
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have a half-life of 8 to 12 years, whereas memory CD8 T cells have
a half-life of 8 to 15 years. The size of the immune system is fixed
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somewhat, but the effector memory CD8 T cell pool can double
without loss of preexisting memory cells.
Human memory T cells express TLR2. If exposed to its ligand

lipopeptide in the presence of either IL-2 or IL-15, they will
proliferate. Thus it is possible that bacterial PAMPs such as the
lipopeptides may promote the long-term survival of memory T cells
even in the absence of persistent antigen.

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