Cow milk allergy: Casein and whey are the two major proteins of human milk and most milk-based formulas. Some (less than 8% of) infants have a true allergy to the cow proteins that are in milk-based formulas.
Infants with true cow milk allergy can develop abdominal pain, diarrhea, rectal bleeding, skin rash, and wheezing when given milk-based formulas. These symptoms will disappear as soon milk-based formula is removed from the diet. (Allergy to cow milk protein is different from lactose intolerance).
The treatment of cow milk protein allergy involves using formulas that contain no cow milk, or using formulas that contain "predigested" casein and whey proteins. The predigesting process breaks the whole proteins into smaller pieces or into amino acids. The amino acids and smaller protein pieces are non-allergenic (do not cause allergy).
Soy protein formulas contain no cow milk, and are reasonable alternatives for infants with true cow milk allergy. However, some infants have allergy to both cow milk and soy proteins. These infants require a formula in which the cow milk protein (casein) has been "predigested" and specific amino acids added to provide a formula that can provide proper nutrition.