Boosting immune system/preventing colds

[Sorsha]

I'm going on a trip that requires a long plane trip next week, and I'm worried about catching a cold. I used to pretty much always do so when I went on vacation (I think it's stress + recycled plane air), until in the last few years I started using this immune boosting supplement ColdFX. But the supplement says not to use while pregnant... I really don't want to end up getting a cold on top of traveling while pregnant!

I plan on taking lots of vitamin C. Does anyone else have any immune boosting/cold preventing strategies that are pregnancy-safe to share?