Natural allergy relief for cats?

Hi, I have a cat with allergy's. His allergy's makes his immune System hard to fight against common colds, so he gets congested and runny noises easily. I was wanting to know if there is any Natural remedies to help the little guy. I heard some herbs could help but wanted to know if anyone else has had experience with this subject, or maybe knows about Natural relief for animals. Thanks for all the help =]