Histamines are a naturally produced chemical that form an important part of the immune system, digestion and act as a neurotransmitter. The histamines for the immune system are produced by mast cells and certain types of white blood cells. As part of the immune response, they affect blood vessels to allow white blood cells and certain proteins involved in the immune response to more easily enter the vessels to fight off any foreign tissue. As a side effect, this results in inflammation, constriction of the bronchial tubes, and the increased production of mucous.