T Helper Cell

T Helper Cell

T helper cells (T_h cells) are a sub-group of lymphocytes, a type of white blood cell, that play an important role in the immune system, particularly in the adaptive immune system. They help the activity of other immune cells by releasing T cell cytokines. They are essential in B cell antibody class switching, in the activation and growth of cytotoxic T cells, and in maximizing bactericidal activity of phagocytes such as macrophages.

Mature T_h cells express the surface protein CD4 and are referred to as CD4+ T cells. CD4+ T cells are generally treated as having a pre-defined role as helper T cells within the immune system. For example, when an antigen presenting cell expresses an antigen on MHC class II, a CD4+ cell will aid those cells through a combination of cell to cell interactions (e.g. CD40 and CD40L) and through cytokines. Nevertheless, there are rare exceptions; for example, sub-groups of regulatory T cells, natural killer T cells, and cytotoxic T cells express CD4 (although cytotoxic examples have been observed in extremely low numbers in specific disease states, they are usually considered non-existent). All of the latter CD4+ T cell groups are not considered T helper cells.

The importance of helper T cells can be seen from HIV, a virus that infects CD4+ cells. Towards the end of an HIV infection the number of functional CD4+ T cells falls, which leads to the symptomatic stage of infection known as the acquired immunodeficiency syndrome (AIDS). There are also some rare disorders that result in the absence or dysfunction of CD4+ T cells. These disorders produce similar symptoms, and many of these are fatal.