Allergies happen because the immune system accidentally recognizes harmless things like food as if they were something dangerous like a virus. Then immune cells like macrophages take a small piece of the allergen to a T cell which activates B cells. B cells make antibodies that mast cells grab and wear. When this happens mast cells can quickly bind to the allergen and release histamine which causes running nose, watery eyes, tight throats, and hives.
