I noticed an interesting phenomenon yesterday: I have been having allergies to grass, but when I begin doing some intense cardio on my spinning bike outside I realized that my allergies went away very quickly. My nose and throat cleared right up and my eyes stopped itching and cleared up. I stayed clear for the rest of the day.
I did a workout again this morning and when I started I was rubbing my eyes and sneezing and had mucus in my nose. Probably 10 minutes into my cardio workout I was totally clear and was clear for the rest of the 65 minute workout.
I guess this is another good reason to exercise!
This may actually be more related to taking your body into a fasted state, I read a couple of studies that say that fasting significantly suppresses hypersensitive immune systems.