Why is histamine sent to where pollen's detected?
The immune system, the body's self defense system, reacts to foreign tissues
that invade it. One of its many, many reactions is to produce histamine. This chemical
acts to bring more blood and lymph fluid to the site of invasion, which acts to bring
more immune cells there to help fight the infection and more blood flow to help carry
away poisons. In some people, but not all, the immune system is hyperactive, and
over-reacts to certain foreign things. This is called an allergy, when it is mild,
or "anaphylactic shock" when it is so severe that it can kill you, as in extreme
reactions to things like bee stings. This overreaction is why histamines get released
when pollen enters. The swelling and fluid caused by blood and lymph vessel swelling
results in the symptoms of stuffy nose and watery eyes. What to do? Take an
anti-histamine, of course.
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Update: June 2012