When someone with an allergy eats drinks touches or sometimes even breathes in an allergen (a food or substance he’s allergic to) his body goes to war. Our immune system — whose job is to combat dangerous germs — sees the allergen as an attacker and fights back. Someone who has an allergic reaction to milk is either reacting to the proteins in the curd (the more solid part of the milk) or the whey (the more liquid part) — or both. An allergy is really your immune system overreacting. When the immune system decides a substance is dangerous even if it’s something harmless (like dust mites or pollen) it produces antibodies like histamine to fight the danger. This overreaction can cause coughing sneezing itchy eyes a rash or hives stomach upset wheezing or more serious breathing problems.