There are a couple of things that you can do to help strengthen her immune system. One is to make sure that she is eating a good quality food. With her age she should still be on a puppy food for large breeds. Alot of people forget that food is more than just keeping them full. I'm sure that you know this but sometimes a reminder helps. Also exercise helps strenghten not only their muscles but their immune systems too. As for adding things to the diet, one product that can do a couple of things is called Missing Link. It adds essential vitamins and minerals that help with the immune system, skin and general health. Using the one that has glucosamine in it will help with keeping her joints healthy and with her size its important to start now. There are a couple of strengths available but I would recommend the veterinary strength just because of her size. Here is the website for the Missing Link that can give you more information on the product.