My Arab has an allergic reaction to...?

Yes! An old school horse I knew used to have this same problem. I think it is not too rare in horses to have this problem. Just like a human allergy; it happens. Some people are allergic to bee's, some are not.. Same with horses and horse flies. The horse I knew was also rugged like that too, and that prevented the lumps from the bites. Don't worry though! It happens in horses!! (: