Trends in histamine research: new functions during immune responses and hematopoiesis

Histamine is a bioamine with multiple physiological activities. In the immune system, it not only mediates the deleterious effects accompanying allergic reactions, but it acts also in a more subtle way by modulating the T helper 1 (Th1)–Th2 balance and possibly hematopoiesis. The histamine required for Th-cell polarization is provided by mast cell or basophil degranulation, as well as being newly synthesized and immediately released by other hematopoietic cells, in response to molecules generated during the immune response. Its global effect depends on the subtype and distribution of histamine receptors on target cells. The recent discovery of a novel H4 receptor, which is expressed preferentially on immunocompetent cells, will have important consequences for the understanding of the regulatory functions of histamine during the immune response