EDIT: the following assumes the OP wants to become an allergist/immunologist MD, not a scientist.
omg, what are you people telling him? this is college he's talking about, not med school electives.
to the OP: major in whatever the hell interests you. Your major will not determine your ability to get into medical school. If you honestly want to do nothing but immunology from college on, then go w/ the other posters advice. If, on the other hand, you just want to become an immunologist as your end-career, then that is not something you will have to worry about until you get to med school. For college, just develop all your interests, and figure out what sorts of courses you both enjoy and will be able to do well in. You will learn plenty of immunology during medical school, your residency, and your fellowship. Someone mentioned doing research in immuno or microbio. That I do agree with, since u're obviously interested in it. That's not something you'll have as much time to do in medical school. But you don't have to major in that area, that's the point.