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Chemokine receptors expressed by T helper cells help recruit cells to specific locations based on their chemoattractant ligands. The polarization of T cells into Th1 and Th2 cells is associated with their expression of different subsets of chemokine receptors. Naive CD4 positive cells that have not been exposed to antigen express CXCR4 and CCR7. The most abundant chemokine receptors on Th1 cells involved in the cellular immune response to microbial agents include CXCR3, CCR1, CCR2 and CCR5, while Th2 cells express CCR2, CCR3 and CCR5. An additional class of T cells termed semi-naive cells may be induced by TGF-beta to express yet another class of chemokine receptors, CCR4 and CCR7. Although the expression of chemokine receptors in different cells is preferred in these cases, it is not absolute or exclusive, and there is overlap in expression between Th1 and Th2 cells. Although CCR7 positive cells have been reported to lack effector action, not be polarized, and to home only to lymphoid tissues, other studies have found CCR7 positive Th1 and Th2 cells and have identified CCR7 positive cells in non-lymphoid tissues. CCR4 appears associated with homing of lymphocytes to the skin, and CCR9 is associated with homing to the small intestine, while other chemokines like CXCR3 and CCR5 are associated with homing toward inflamed tissues more broadly. The expression of different chemokine receptors by subsets of T helper cells plays an important role in the migration and homing of these cells in tissues, and targeting of the immune response to specific cells. (This definition may be outdated - see the DesignNote.)
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