A simple explanation for an allergic reaction may be the over-reaction of the bodies defenses against whatever it perceives to be an enemy to the body...even if it is something that many other people that you know do not have an allergic reaction too.
When it comes to food, my college studies included learning about both food sensitivities and food allergies. Before my studies I didn't know there was a difference between the two. But even though what is happening in the body may be different, both can have an similar discomfort to the person suffering from them.
Just last night I was online looking for a Homeopathic remedy for the flu suggested by my cousin, who is a Homeopath. During my search, I also found this homeopathic remedy for allergies.