For cats, just feed them egg from chickens that live with cats, as the egg has antibodies against the allergen in cat saliva, and the egg will cause those to get neutralized before the cat spreads them to its fur.
There are commercial products for this that have such egg powder.
It seems that is basically the same mechanism as the Purina Live Clear cat food? That one we tried, and did not notice a definitive effect. What did help was keeping the cats out of the bedroom, and later being pregnant removed the allergy so far almost completely for her ;)
There are commercial products for this that have such egg powder.