<P> CD4+ lymphocytes, also called "helper" or "regulatory" T cells, are immune response mediators, and play an important role in establishing and maximizing the capabilities of the adaptive immune response . These cells have no cytotoxic or phagocytic activity; and cannot kill infected cells or clear pathogens, but, in essence "manage" the immune response, by directing other cells to perform these tasks . </P> <P> Helper T cells express T cell receptors (TCR) that recognize antigen bound to Class II MHC molecules . The activation of a naive helper T - cell causes it to release cytokines, which influences the activity of many cell types, including the APC (Antigen - Presenting Cell) that activated it . Helper T - cells require a much milder activation stimulus than cytotoxic T cells . Helper T cells can provide extra signals that "help" activate cytotoxic cells . </P> <P> Classically, two types of effector CD4+ T helper cell responses can be induced by a professional APC, designated Th1 and Th2, each designed to eliminate different types of pathogens . The factors that dictate whether an infection triggers a Th1 or Th2 type response are not fully understood, but the response generated does play an important role in the clearance of different pathogens . </P> <P> The Th1 response is characterized by the production of Interferon - gamma, which activates the bactericidal activities of macrophages, and induces B cells to make opsonizing (coating) and complement - fixing antibodies, and leads to cell - mediated immunity . In general, Th1 responses are more effective against intracellular pathogens (viruses and bacteria that are inside host cells). </P>

Where are cells of the adaptive immune system generated