Causes

All true food allergies are caused by an immune system malfunction. If you have milk allergy, your immune system identifies certain milk proteins as harmful, triggering the production of immunoglobulin E (IgE) antibodies to neutralize the protein (allergen). The next time you come in contact with these proteins, IgE antibodies recognize them and signal your immune system to release histamine and other chemicals, causing a range of allergic signs and symptoms.

There are two main proteins in cow's milk that can cause an allergic reaction:

• Casein, found in the solid part (curd) of milk that curdles
• Whey, found in the liquid part of milk that remains after milk curdles

You or your child may be allergic to only one milk protein or to both. These proteins may be hard to avoid because they're also in some processed foods. And most people who react to cow's milk will react to sheep's, goat's and buffalo's milk. Less commonly, people allergic to cow's milk are also allergic to soy milk.

Food protein-induced enterocolitis syndrome (FPIES)

A food allergen can also cause what's sometimes called a delayed food allergy. Although any food can be a trigger, milk is one of the most common. The reaction, commonly vomiting and diarrhea, usually occurs within hours after eating the trigger rather than within minutes.

Unlike some food allergies, FPIES usually resolves over time. As with milk allergy, preventing an FPIES reaction involves avoiding milk and milk products.