Histamines

I had been under the impression that histamines were produced by the body as a result of an allergic reaction-- hence, the use of antihistamines such as loratadine or diphenhydramine (the main ingredients of Claritin and Benadryl, respectively).

Turns out this is not, strictly speaking, the case. Histamines are produced as a result of an allergic reaction, but they can be produced by other causes as well. According to Stedman's Medical Dictionary, histamine is "A vasodepressor amine derived from histidine by histidine decarboxylase and present in ergot and in animal tissues. It is a powerful stimulant of gastric secretion, a constrictor of bronchial smooth muscle, and a vasodilator (capillaries and arterioles) that causes a fall in blood pressure. Histamine, or a substance indistinguishable in action from it, is liberated in the skin as a result of injury. When injected intradermally in high dilution, it causes the triple response."

Yeah, I didn't understand much of that either.

Wikipedia is a little clearer:  histamines are produced by the body as part of an immune response, which can be prompted by a variety of things, although allergens are the most common.  When absorbed by the four types of histamine receptors, the histamines produce inflammation, which (depending on the location and type of receptor) will in turn produce the itching, swelling, runny nose, bronchoconstriction (tight airways), etc. that we associate with allergic reactions.  They are also involved in other parts of the body in gastric acid and neurotransmitter production.

An interesting side note is that antihistamine drugs don't work by stopping the production of histamines, as a literal interpretation of the name might suggest.  Instead, they flood the receptors that would otherwise absorb the histamines, thus preventing the reaction from completing.  So instead of being anti- histamine production, they're anti- histamine reaction.