I want to know how you can boost and or keep your immune system at its maximum. What supplements and foods should I eat for this?

The reason behind this question is that i am going to be getting prolotherapy in 3 weeks and they say the stronger your immune system is, the better it works. I’m going to see the best prolotherapist in the midwest US.

My immune system is ok i guess. I only get sick about twice a year. However, it takes me about 2 to 3 weeks sometimes to get rid of colds.