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Allergy to cow's milk is the most common food allergy among infants and young children, affecting about 2.5 percent of kids under the age of three. Milk allergy is not the same as being lactose intolerant. The two conditions aren't even related. Lactose intolerance means that a person has a hard time digesting the sugar that's in milk. A milk allergy (like all allergies) is an overreaction of the immune system in response to protein and it can be deadly. Most children eventually outgrow their milk allergy but blood tests that measure the presence of milk antibodies can be used to check.