HELP i need to boost immune system in my mare

Long story short- I have an amazing TB mare, that I breed from. She's just had a tumour removed from her 3rd eyelid and in danger of loosing whole eye if she gets another. She's gets the best care, nutrition etc, but in the 2 years I've had her has never looked 100%.

I study nutrional medicine and how diet effects are health etc

Bad immune systemms in humans make you more prone to cancer etc, so assuming its same in horses

I want a good, preferably natural anti-oxidant type supllement to help boost this mare so she can fight off these reoccuring tumours

Suggestions???????