CD4+ T cells play an integral role in adaptive immune responses. When a pathogen is detected, information is communicated to T cells through antigen presentation. Following activation, naive CD4+ T cells differentiate into one of several lineages of T helper cells (Th1, Th2, Th9, Th17, or Th22), depending primarily on the antigen, the strength of the TCR signal, and the cytokines present in the surrounding extracellular environment. Differentiation of each T cell subset is associated with the expression of specific transcription factors followed by secretion of a defined array of cytokines that orchestrate a directed response to the antigen. The actions of T helper cells are balanced by regulatory T (Treg) cells, a subpopulation that specializes in suppression of T cell-mediated immune responses. Failure to activate an appropriate T cell response can lead to chronic infection, while exaggerated T cell responses can cause excessive tissue damage and are associated with inflammatory and autoimmune diseases.