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Is there a general rule for how antihistamines - as a class of molecules - affect blood pressure? Or is their no such generalization for either the affect they have on blood pressure or for all antihistamines in general, that is, maybe certain ones affect blood pressure differently. If their is such a rule, what is it and how does this occur, biochemically speaking?

In general they do not affect blood pressure.

Now, if someone were to have a severe allergic reaction, sometimes blood pressure can fall to dangerous levels, and antihistamines can to some extent prevent this.

See the Merck Manual (now free online)

http://www.merck.com/mmpe/sec13/ch165/ch165c.html?qt=antihistamine&alt=sh#sec13-ch165-ch165c-288

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