Hello~One culprit that my naturopath shared was a weakened immune system and also weak adrenal glands. I had lots of food allergies when younger, mine were due to stress and over-worked adrenals. I took a good, high potency B complex and this seemed to help.
So, a little more direction with your question would be helpful. Are you talking about seasonal allergies? Allergies around the home? Food allergies?