If the object of the game, is to protect their immune system, we give L-Lysine, I have no idea, what the dose for doggies is, and you get it in the human vitamin section, unless, they make it just for doggies in a product. You want the one, with NO fillers.
Another thing you can give is Omega 3 and Omega 6, this too, is an antioxidant. You want the one, made for pets, and a good quality one, you cannot give the human one.
http://www.onlynaturalpet.com/search.aspx?avs|Pet+Type=Cats&avs|Department=Supplements&avs|Health+Condition=Skin+%26+Coat+Conditions
Found this too.
http://truthaboutpetfood2.com/heads