Can you improve your immune system by drinking orange juice?

Not really: Review of current literature demonstrates a overall inconsistent set of data to suggest that orange juice, or its constituents (like vitamin c), have any broad sweeping or amazing effect on immune function. Most review papers have demonstrated that while vitamin c may reduce some symptoms of acute infections, it has not been shown to reduce the number of infections or work as prophylaxis for cold.

Vit C: You can't hurt yourself and you will get an increased amount of vitamin c, but one cannot truly say that orange juice, into and of itself, actually improves one's immune system.