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 found on professional antigen-presenting cells (APCs).
Once bound to an antigen, the TH cell proliferates and differentiates into activated TH cells and memory TH cells.
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. The decrease in number of CD4+ T cells is the primary mechanism by which HIV causes AIDS.