A helper (or T_H) T cell is a T cell (a type of white blood cell) which has on its surface antigen receptors that can bind to fragments of antigens displayed by the Class II MHC molecules found on professional antigen-presenting cells (APCs).

Once bound to an antigen, the T_H cell proliferates and differentiates into activated T_H cells and memory T_H cells.

• Activated T_H cells secrete cytokines, proteins or peptides that stimulate other lymphocytes; the most common is interleukin-2 (IL-2), which is a potent T cell growth factor. Activated, proliferating T_H cells can differentiate into two major subtypes of cells, Th1 and Th2 cells. These subtypes are defined on the basis of specific cytokines produced. Th1 cells produce interferon-gamma and interleukin 12, while Th2 cells produce interleukin-4 , interleukin-5 and interleukin-13.
• Memory T_H cells are specific to the antigen they first encountered and can be called upon during the secondary immune response .

Most T_H cells have present on the cell surface the protein CD4 , which is attracted to portions of the Class II MHC molecule. This affinity keeps the T_H cell and the target cell bound closely together during antigen-specific activation. T_H cells with CD4 surface protein are called CD4+ T cells. The decrease in number of CD4+ T cells is the primary mechanism by which HIV causes AIDS.