There things you can do to improve your immune system, aside from modifications ot your diet.
1. Take glutamine. It could also help you with your sleep as well, if you get too many nights where it takes 1-2 hours or more to nod off, versus 5-60 minutes.
2. Take zinc pills.
3. Take vit c, which could be done with 500mg amount in the morning and possibly another one last thing at night, or alternatively change that dose to 1g, but if it feels a bit harsh when you take it switch to buffered as buffered c is supposed to be a bit more gentle for people who experience a slight issue with regular c.
You may want to google these though to be on the safe side and for your own piece of mind. The best kind of sites to look at would ones like wisegeek, medicinenet, webmd, medline plus and Linus Pauling Institute to name a few and Linus Pauling won two nobel prizes I think for peace and chemistry, so you can trust anything that site says with your life in honest opinion.
Most if not all of those have good accreditation's like hon@ code and ADAM accreditation, which means they have been properly assessed in credible ways for the credibility of their info.
In relation to the diet there are some really good sources of zinc and vit c.
Admittedly whilst oysters are the best source of zinc, they are pricey, but chicken is always a good one and many other foods have zinc in like shellfish and beans for example.
As for vit c, pink guava is the highest at around 165mg, then broccoli and then things like oranges, tomatoes and potato, pink guava should be slightly better than normal guava as they contain lycopene, a carotenoid that can convert to vit a, (retinol-a) and helps destroy free radicals reducing oxydative stress and promoting more youthfulness in the skin for one thing, plus tomatoes are good at fighting cancer as they are a good source of vitamin c.
So there's a few things to think about
GOOD LUCK