Tcell priming

CD4 and CD8 cells can be primed in the T zones and undergo IL-2-dependent proliferation. This increases the number of antigen-specific T cells and alters the phenotype of the T cell. The level of adhesion molecules changes, as LFA-1 (CD11a, CD 18 heterodimer), VLA molecules (heterodimers of CD29 and CD49) CD44 and CD2 are among those that may be up-regulated. At the same time there is downregulation of some receptors such as CD62L (L-selectin), which is required for the entry of recirculating B and T cells from the blood to secondary lymphoid organs. The important effect of these changes is the expression of surface molecules that allow primed T cells to enter non-lymphoid tissues, particularly those that drain lymph into the lymphoid organs where their precursors were primed.

Mast cell

Mast cell

NK cell

Macrophage

IL-3 TNF-a

IL-2

ILF-y

IL-3 TNF-a

IL-2

ILF-y

NK cell

Macrophage

Figure 20.15 Maturation pathways of CD4+ T helper cells. CD4+ T cells that have been activated by antigen acquire the capacity to produce cytokines. The cytokines that they produce depends on the environment in which they are activated. Two main types of cytokine-producing TH cell are recognized - TH1 and TH2 cells. The cytokines produced by TH1 cells tend to promote further TH1 cell formation and inhibit TH2 cell formation, and IL-4 produced by Th2 cells promotes further differentiation towards TH2 cells. TH1-promoting cytokines are also produced by activated macrophages, dendritic cells and NK cells, whereas mast cells produce IL-4.

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