Dear Etiquetteer: We value our friends almost as much as we value our cat, and don't wish to cause discomfort - or, on the other hand, exclude people. How should we decide the question of whether or not to invite persons with cat allergies to our abode?
By letting the Afflicted decide to accept or decline your invitation themselves. Allergies can sometimes be controlled sufficiently by medication, suppressing the allergic reactions enough to allow party attendance. Indeed, Etiquetteer himself has acquired with age an allergic reaction to Certain Felines, and knows to apply antihistamines before arriving.* But this is very much an individual choice, and if your Afflicted Friends feel it necessary to send regrets, you must accept them with compassionate understanding.
The allergies of your Afflicted Friends to your Feline Familiar, however, provide you the opportunity to entertain them in other ways, such as a backyard barbecue or a museum tour or a restaurant dinner. You're still the hosts entertaining your friends, but in locations that eliminate or minimize their discomfort.
*Etiquetteer is not a doctor, and doesn't even play one on TV. Consult your physician about possible solutions to allergic reactions that will work best for you.