Q: Last month I broke out in hives after eating oysters. I had a blood test, which came back negative for a shellfish allergy. Why does my doctor still want me to do a food challenge?
A: Allergic reactions occur when your body's immune system overreacts against a harmless substance -- in your case, possibly, shellfish.
Food allergies can cause a variety of symptoms that range from mild to life-threatening. When a person is allergic to something, like oysters, it's not to the whole oyster. The body is reacting to a substance called an allergen. This chemical is a part of the oyster, just one of many substances that make it up.
When the oyster is digested, it is broken into tiny pieces in your stomach; those pieces include the allergen. We don't understand well why it is that the immune system of an allergic person reacts so violently when it sees a particular allergen.
The allergen can't damage us, but the immune system regards it as a threat and starts attacking. It's the "shrapnel" from the immune system attack that causes the symptoms we experience during an allergic reaction.
A skin-prick test is usually the first test used to diagnose a food allergy. The doctor begins by pricking the skin on your back or on the inside of your forearm with a needle. He or she then places a small amount of the suspected allergen (or allergens) onto the pierced skin.
If you are allergic to the substance being tested, within 15 minutes an itchy, swollen, red spot will develop on your back or forearm. Your immune system has spotted the allergen under your skin and started to attack it.
Less commonly, doctors use a blood test for diagnosis, as in your case. A small blood sample is analyzed to measure the levels of IgE antibodies to the food being tested. (IgE is the type of antibody that causes most allergic reactions.) A certain level of IgE antibody in your blood indicates an allergy.
It would seem that a positive blood or skin test should be enough to diagnose a food allergy, or a negative test to rule one out. But it's not that simple.
Positive tests indicate that the IgE antibody is present in your blood, but the tests can't confirm whether or not you will experience an allergic reaction when you eat the food.
That's why the most reliable test is a food challenge: You eat small amounts of a suspected food until you begin to have an allergic reaction to it. If you are able to eat a normal serving without an allergic reaction, your doctor can safely rule out an allergy to that food.
Food challenges can, by their nature, result in an allergic reaction. That's why they should always be conducted by an experienced clinician in a medical facility equipped to treat a life-threatening allergic reaction. These are rare, but they happen. Fortunately, with the right medicines readily at hand, the reaction can be stopped.
• Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com.