T-helper cells (Th cells) are a subset of αβT-cells that usually express the CD4 co-receptor and have a major role in controlling and regulating the immune system by providing ‘help’ to other white blood cells.
Th cells are vital to human immune responses because they orchestrate the immune system by controlling other T-cell subsets, B-cells and innate immune responses. The Th cell response is defined by two distinct pathways involving two different subtypes of Th cell; Th1 and Th2 cells. Classically, Th1 cells are targeted towards intravesicular pathogens such as bacteria and parasites via the activation of infected macrophages, whilst Th2 cells invoke antibody production in B-cells, which neutralise extracellular pathogens and toxins. Th cell activation leads to the induction of a number of pathways that can result in B-cell antibody production and immunoglobulin class switching, and macrophage action via both direct interaction and through the release of soluble factors.
Th cell responses to disease are initiated through the interaction between the TCR, on the surface of a T-cell, and protein fragments derived from ‘foreign’ invaders that are presented by pMHCII on the surface of infected cells. Once a Th cell has identified a cell expressing a ‘foreign’ pMHCII, it makes soluble molecules that alert the rest of the immune system and help it eliminate the foreign element. Although T-cell activation is dominated by the TCR/pMHC interaction, it also involves the activities of the CD4 coreceptor. Understanding these mechanisms that underlie Th cell antigen recognition and Th cell activation are vital for understanding and treating disease.