Firstly, make sure she has a high protein diet. Then you can add a little fish oil to her food for Omega 3 & 6 fatty acids. You can also give your cat probiotics to promote a healthy gut. Purina sells Forti Flora for example. You can also find colostrum in capsule or powder form, but this is mostly good for kittens' immunity. This is just coming from someone who has owned cats most of her life, but honestly, if you're concerned about your pet's immune system, you should probably see a vet.