T helper cell

The T helper cells (T_h cells) are a type of T 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. These cells help suppress or regulate immune responses. 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. Such 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 cells, and cytotoxic T cells express CD4. All of the latter CD4⁺ T cell groups are not considered T helper cells.