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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.

However, histamines are also involved in allergies and most autoimmune conditions. A runaway histamine response is the cause of anaphylactic shock.

Antihistamines are a class of pharmaceuticals that either reduce the amount of histamines or inhibit their action in the body.  They are most commonly used to treat allergies.  

Histamine at Wikipedia