Helper T cell

A helper (or TH) 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 of infected somatic cells.

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

   * Activated TH cells secrete cytokines, proteins or peptides that
     stimulate other lymphocytes; the most common is interleukin-2 (IL-2),
     which helps B cells differentiate into plasma B cells.
   * Memory TH cells are specific to the antigen they first encountered and
     can be called upon during the secondary immune response.

Most TH 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
TH cell and the target cell bound closely together during antigen-specific
activation. TH cells with CD4 surface protein are called CD4+ T cells.

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