How to reinforce my immunity?

I discovered from my last blood test that I have an WBC=4.0 !!! I do eat properly,fruits vegetables and chicken or fish a loooot of salad. don't understand why can it be so low. what can I do? my doctor told me to eat well 3times a day and a looot of fruit and sleep well.. anything else to do? and what can lower immunity and how to raise it?