Milk allergy is far more common in children than in adults. 90% of milk allergic children lose the allergy by the age of three. It is extremely uncommon for adults to have milk allergy. Many adults, however, cannot tolerate milk because of an inborn deficiency in the ability to break down the milk sugar, lactose (lactose intolerance).
The majority of milk allergic children have two or more symptoms. About 50-70% have skin symptoms, 50-60% have symptoms from the stomach and gut, and 20-30% have symptoms from the airways. Severe and life threatening symptoms may occur in 10% of the children.
Milk contains several different allergens. People are usually allergic to more than one kind of milk protein.
The proteins from cow's milk are very similar to those from goats and sheep, and can cause the same sorts of reaction in cow's milk-allergic subjects. Thus goat's or sheep's milk cannot be used as a cow's milk substitute in allergic individuals.