Naive T cell
Naive T cell refers to 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 to not 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+CD62LlowCD69+), 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. As there are a limited number of T-cells in the body, eventually the amount can be taken up by memory cells for pathogens encountered in the past that may no longer be needed in comparison to more widely spread modern strains of pathogens.
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