A milk allergy is cause by a dysfunctional immune system. Your immune system identifies milk proteins as an "invader" and harmful to your body. As a result, your immune system produces and releases immunoglobulin E (IgE) antibodies into your bloodstream to attack and neutralize the "harmful" milk proteins. Your immune system also reacts this way to a virus or infection in your body. An infection or virus, however, is a real "invader" that is harmful to your system. The next time you consume milk or milk products, your immune system signals the immunoglobulin antibodies to release histamine. Histamine is the main culprit for all those nasty food allergy symptoms, including hives, itchiness, sneezing, watery eyes, nausea, diarrhea, and even anaphylactic shock [source: Mayo Clinic].
There are two milk proteins that typically cause an allergic reaction: casein and whey. Casein is the protein found in the solid part of milk, known as milk curd. Whey is the protein found in the liquid part of milk [source:Mayo Clinic]. Approximately 80 percent of milk protein is casein. As you may already suspect, casein is the main protein in cheese; in fact, the harder the cheese, the greater the amount of casein. Your milk allergy may be the result of an allergy to one or more milk proteins [source: Food Reactions].
There are several risk factors for developing a milk allergy. First, if you have a family history of food allergies or other allergies, such as hay fever, you are at increased risk of developing a milk allergy. Second, infants and children are at greater risk of developing a milk allergy compared to adults, especially infants and children with other pre-existing food allergies. Third, children with a certain skin condition called atopic dermatitis are at risk of developing a milk allergy [source:Mayo Clinic].