There are extreme training methods in boosting your immune system. Ive heard extremely cold water dives with sauna has been proven to improve immune system response, but I would not recommend that straight off the bat.
The easy steps are as follows:
1)Dont smoke
2)Sleep adequately
3)Eat a diet high in fruits and vegetables
4)Exercise regularly
5)Maintain a healthy weight
6)try minimizing stress
7)Take steps to avoid infection, good hygiene, wash hands.