DEAR MAYO CLINIC: My one-year-old son was diagnosed with a milk allergy, but he is not lactose intolerant. How are the two different?

ANSWER: Although they may seem similar, being allergic to milk and being lactose intolerant are two unrelated medical conditions. A milk allergy involves the body's response to a protein found in milk. Lactose intolerance involves a carbohydrate found in dairy products.

Foods are made up of protein, carbohydrate and fat. All of these are necessary parts of a person's diet. Protein is found mainly in dairy products, eggs, meats, nuts, peanuts, fish, seafood and beans. However, even grains, fruits and vegetables have some small amounts of protein. When someone has an allergic reaction, it's almost always the result of the immune system responding abnormally to a specific food protein.

With a milk allergy, the immune system abnormally identifies certain milk proteins as harmful. That triggers the production of immunoglobulin E, or IgE, antibodies against the proteins. When the IgE antibodies come in contact with the proteins, the antibodies send a signal to the immune system to release histamine and other body chemicals. Those chemicals cause the body to produce an allergic reaction.

For people with a milk allergy, the allergic reaction can occur from within minutes to about three hours after the protein is consumed. A milk allergy can cause skin redness and itching, hives, swelling, difficulty breathing, vomiting and diarrhea. In severe cases, an allergic reaction may lead to anaphylactic shock, a life-threatening condition that can cause low blood pressure and loss of consciousness.

The only way to prevent an allergic reaction in people who have a milk allergy is to avoid milk and milk proteins altogether. If an allergic reaction does occur, an injection of the drug epinephrine may be needed to control symptoms and, in severe cases, to prevent or treat anaphylactic shock.

In contrast to a milk allergy, lactose intolerance does not involve food proteins or the immune system. Instead, it results from a problem with a carbohydrate -- lactose -- that is found in milk and other dairy products.

When you drink milk or eat dairy products, enzymes in the gut digest lactose, so the body can make energy. In people with lactose intolerance, a certain enzyme, called lactase, is missing from the body. When that person eats dairy products, the body has no way to break down the lactose.

This leads to symptoms such as diarrhea, nausea, abdominal cramps, bloating, gas (flatus), and rarely, vomiting. Typically, these symptoms appear about two to four hours after eating a dairy product. The symptoms produced by lactose intolerance can be uncomfortable. But unlike those of a milk allergy, they are very rarely dangerous.

No treatment is available to cure lactose intolerance. And at this time there is no way to increase the body's production of the lactase enzyme. The most effective way for people with lactose intolerance to get relief from their symptoms is by eliminating or reducing the amount of dairy products they eat. They also may be able to use dairy products that have reduced levels of lactose or are lactose-free.

Some people with lactose intolerance benefit from using over-the-counter tablets or drops that contain the lactase enzyme to help digest dairy products. Not everyone with lactose intolerance is helped by these products, however. -- Nancy Ott, M.D., Pediatric Allergy and Immunology, Mayo Clinic, Rochester, Minn.

(Medical Edge from Mayo Clinic is an educational resource and doesn't replace regular medical care. E-mail a question to medicaledge@mayo.edu. For more information, visit http://www.mayoclinic.org.)