Preventing cat allergies?

im allergic to cats, get the full puffy itchy eyes, sneezing non stop thing. i take phenergan, which is an antihistamine, once ive reacted but i was wondering if there's anything i can take to prevent it. so say if im going to a friends house and i know in advance that they have a cat, is there anything that can either prevent or even prolong getting the reaction?

i know staying away from cats is the obvious thing but its normally the hair floating around that does it, not the actual cat, so sometimes its a bit hard to just avoid them..

thanks for any suggestions!

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