There are many things you can do to improve immunity.
Avoid sugar and cafferin
Eat more multicoloured fruit and vegetables
Eat enough protein
Stay hydrated with 1.5- 2 lites of pure water daily
Get at least 8 hours of sleep per night
Probiotics are good and you should also take a good antioxidant complext containing A,C,E and the minerals selenium and zinc for about three months. I would also advise you to get your vitamin D levels checked and start supplementing if you are low. The best vitamin D is D3 and it can work wonders for your immune system. Don't stay on D3 long term though. Get tested and stop taking it once your levels return to normal.
I have clients coming to me to help with immunity and each case is slightly different and requires a different approach..
If the above doesn't work then if may be a good idea to see a Nutritionist who can look into your specific case and medical history and recommend appropriate diet and supplements.
Wishing you the best of health and vitality.
Head Nutritionist and Founder
Nuriheal Nutrition and Health Clnicshttp://www.nutrheal.net
0207 474 6880