What can I rake to build my imune system?

Not just one thing. Best ways to improve your immunity would be to eat a healthy balanced diet, get at least 7 to 8 hours of sleep each night, and consider probiotic supplements or yogurt. Avoid things that weaken your immune system such as cigarette smoke.
Take care of 4 thing. 1) sleep 7 hrs per night 2) exercise 3) eat nutritionally dense foods 4) control stress.
Be healthy. There is no such thing as "boosting" the immune system. Staying healthy with a good, well-balanced diet, enough rest, and minimizing stress is the best way to do that.