citrus of course, berries, tropical fruits, lightly steamed brocolli. just make sure they are organic or you are wasting your time and try and buy as local as possible. the longer it is shipped the less nutrition it will still have! and if it isn't organic you are just buying dead food
also fasting boosts immune system bigtime. whether it is intermittent or prolonged, you should be doing it here and there. prolonged fasts cause bad and weak cells to die, then you develop fresh troops which boosts the immune system more than anything you can do