Helper T Cells

CD4⁺ helper T cells play a central role in directing adaptive immune responses. They develop in the thymus and differentiate into more specialized subsets (Th1, Th2, Th9, Th17, or Th22) following activation by antigen-presenting cells (APCs). Differentiation into a specific subset depends on the antigen, the strength of the TCR signal, and cytokines present in the extracellular milieu. The cytokines present in the extracellular environment activate intracellular signaling pathways that regulate the expression of subset-specific receptors, transcription factors, and cytokines, which are commonly used to identify different helper T cell subsets. The molecules that the different subsets secrete are not only intended to eliminate the invading pathogen, but they also play an important role in directing the activities of other immune cell types. As a result, unregulated expression of the cytokines secreted by different helper T cell subsets contributes to the pathogenesis of multiple inflammatory and autoimmune disorders.