Before DH did allergy shots, he just couldn't stay indoors for very long in homes with dogs. He'd have to use different inhalers, be on maintenance allergy and asthma meds, and still spend most of the visit outdoors. Made wintertime rough.
When staying with my parents back when they had a house, we put filters over the furnace vents (or closed them and used extra blankets) and added a HEPA filtration unit to the room. And we kept the dogs out of that room (my old room). So he could stay up there if he didn't want to be outside. Dog dander isn't as sticky as the cat allergens, so vacuuming and wiping down the walls can help quite a bit. This is a guy who had issues just being outdoors at a dog park, though, so it was rather rough.
When my allergy bucket is full, I can't really go in homes with cats. Normally, my meds will give me about two hours before a bad asthma attack. Right now, it might be five minutes. It's just how it is. I'll probably need my inhaler today because cat-living friends are coming to my house. :/