hi and welcome to our cat forum. I will include a rather lengthy thread for you to read, as you will see its a common and complex problem.
One culprit is the litter, thats the easiest place to start, change that, particularly if its a high scented one....go to unscented or preferably even a totally different type.
If feeding dry food stop and buy canned food, dry food has many more chemicals that can contribute to allergies.
can be also an allergy to fleas,ticks,dust pollen...as with my cat.
allergies in ppl show up by sneezes or runny eyes, in cats they show up as a skin problem, itch or sores.
as with my Sami, he's hypersensitive to chicken, and most cat foods have a chicken base. try switching to some of the alternatives on the thread below....at least away from chicken. Venison is the best choice if kitty will eat it.
good luck, read thru the thread, if theres more we can help with please post again and thanks for helping this lady and kitty out.
please send us an update.