A milk allergy is an abnormal response by the body's immune system to two main proteins in cow's milk that can cause an allergic reaction: Casein, which is found in the solid part (curd) of milk that curdles, and Whey, which is found in the liquid part of milk that remains after milk curdles. Cow's milk is the usual cause of milk allergy, but milk from sheep, goats, and buffalo also can cause a reaction. Milk allergy is one of the most common food allergies in children. Fortunately, most children outgrow a milk allergy by age 3. An allergic reaction usually occurs minutes to hours after consuming milk. Signs and symptoms of a milk allergy range from mild to severe and can include wheezing, vomiting, hives, and digestive problems. Rarely can a milk allergy cause anaphylaxis.