T Helper Cell

Helper T (Th) cells play an important role in adaptive immunity and are needed for almost all adaptive immune responses. They are crucial for defense against both extracellular and intracellular pathogens. The functions of helper T cells are activation of cytotoxic T lymphocytes (CTLs) to kill infected target cells, and activation of B cells to secrete antibodies and macrophages for the destruction of ingested microbes. Helper T cells can only function when activated to effector cells and can be induced by binding to class II MHC protein on activated dendritic cells (DCs).

Proliferating helper T cells (CD4+ cell subtype) that develop into effector T cells differentiate into either of the two major subtypes of cells, Th1 or Th2 cells, after activation in a peripheral lymphoid organ. Naïve Th cells activated in the presence of IL-12 develop into Th1 cells and in the presence of IL-4 develop into Th2 cells. The effector Th cells activate their target cells by producing a combination of membrane-bound and secreted costimulatory proteins.

Th1 cells secrete IFN-γ to activate macrophages (which attack pathogens) and CTLs (which kill infected cells) and to induce B cells to switch Ig class. Th2 cells secrete IL-4, IL-5, and IL-13 to induce B cells to switch Ig class, from making IgM and IgD to making IgE antibodies. Th1 and Th2 cells can help activate B cell proliferation and differentiation into antibody-producing effector cells or memory cells and help DCs activate T cells. Th2 cells interact with mast cells, basophils, and eosinophils and are involved in the allergic reactions.

CD: cluster of differentiation; IFN-γ: interferon gamma; Ig: immunoglobulin; IL: interleukin; MHC: major histocompatibility complex; Th1: type 1 T helper; Th2: type 2 T helper.


  • Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P. Molecular Biology of the Cell, 4th edition. New York: Garland Science. 2002.