Cow's milk is one of the most frequent food allergens. Milk allergy is the immune system's response to one or more of the proteins found in cow's milk. There are many protein allergens in cow's milk that cause allergic reactions. Casein and whey are the two main components. The curd that forms when milk is left to sour, is called casein. The watery part which is left after the curd is removed is called whey. Casein accounts for 80% of the protein in milk and is the most important allergen found in cheese. The harder the cheese, the more casein it contains. Whey accounts for the other 20% of milk protein. It consists of two main allergenic proteins - alpha-lactalbumin and beta-lactaglobulin.

Apart from lactose intolerance, many children, as well as adults, have a sensitivity, intolerance, or outright allergy to cow's milk. Children may outgrow the allergy, yet continue to manifest symptoms associated with sensitivity or intolerance.