Naive T cell
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A naive T cell or Th0 cell[1] is a T cell that has differentiated in bone marrow, and successfully undergone the positive and negative processes of central selection in the thymus. A naive T cell is considered mature, but is distinguished from activated T cells or memory T cells, as it is thought not to have yet encountered cognate antigen in the periphery.
Naive T cells are commonly characterized by the surface expression of L-selectin (CD62L); the absence of the activation markers CD25, CD44 or CD69; and the absence of memory markers, such as the edited CD45 isoforms. In the naive state, T cells are thought to be quiescent and non-dividing, requiring the common-gamma chain cytokines IL-7 and IL-15 for homeostatic survival.
Naive T cells are able to respond to novel pathogens that the immune system has not yet encountered. Recognition by a naive T cell clone of its cognate antigen results in the initiation of an acquired immune response. In the ensuing response, the T cell acquires an activated phenotype (CD25+, CD44+, CD62Llow, CD69+), and may further differentiate into a memory T cell.
Having adequate numbers of naive T cells is essential to an immune system for it to be able to adapt to new pathogens experienced in life.
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