Food Allergy Overview
For someone with a food allergy, eating or swallowing even a tiny amount of a particular food can cause symptoms such as skin rash, nausea, vomiting, cramping, and diarrhea. Because the body is reacting to something that is otherwise harmless, this type of allergic reaction is often called a hypersensitivity reaction. Rarely, a severe allergic reaction can cause a life-threatening set of symptoms called anaphylaxis, or anaphylactic shock. Although about 25% of people believe they have a food allergy, only about 2.5% of adults and about 6-8% of children, mainly younger than 6 years, have true food allergies. The rest have what is known as food intolerance—an undesirable reaction to a food that does not involve the immune system.
It is easy to confuse food intolerance with food allergy because they can have similar symptoms. With food intolerance, however, you usually get only mild symptoms such as an upset stomach.
A common example of food intolerance is lactose intolerance—a condition in which a person is missing a certain enzyme necessary to digest dairy proteins. The result is loose stools, gas, and nausea after consuming dairy products such as milk or cheese.
Another example of food intolerance is reaction to MSG. MSG, or monosodium glutamate, is a white-colored additive used to enhance the flavor of food. It is a fermented mixture of glutamic acid, sodium, and water and is used mainly in Asian cooking. Over the last 30 years, side effects from MSG have been related to its use in Chinese food and referred to as the Chinese restaurant syndrome. In this syndrome, MSG was suggested as the cause of the symptoms following a Chinese meal. In 1995, a new term was coined, the MSG symptom complex, to include all the reactions that were reported to be related to MSG. These reactions are not a true food allergy, and the exact cause of the reactions is unknown.
Food Allergy Causes
An allergic reaction occurs when the body's immune system overreacts to an allergen, in this case a food protein.
The white blood cells produce an antibody to this allergen, called immunoglobulin E or IgE.
When this antibody comes in contact with the particular food protein, it promotes production and release of certain chemicals called "mediators." Histamine is an example of a mediator.
These mediators act on various parts of the body, mainly the skin, throat, airways, intestines, and heart.
The effects of the mediators on organs and other cells cause the symptoms of the allergic reaction.
Any food has the potential to trigger an allergic reaction, but a few foods account for most food allergies. In fact, about 90% of food allergies are triggered by one of these 8 foods:
Eggs
Milk
Wheat
Soy
Peanuts
Tree nuts
Fish
Shellfish
Generally, people who have allergies react to only a few foods. Occasionally a person who is allergic to one food also may be allergic to other related foods. This is called cross-reaction. Common examples:
Allergy to peanuts - Cross-allergies to soybeans, green beans, and peas
Allergy to wheat - Cross-allergy to rye
Allergy to cow's milk - Cross-allergy to goat's milk
Allergy to pollen - Cross-allergies to foods such as hazelnuts, green apples, peaches, and almonds
People who have a history of other allergies, such as eczema or asthma, are particularly prone to having a reaction to a food. They are also more likely to have a more severe reaction.
Food Allergy Symptoms
A person with a food allergy can have symptoms beginning as soon as 2 minutes after eating the food, but reactions may take 1-2 hours to appear. Occasionally, symptoms abate quickly, only to recur in 3-4 hours.
The most common symptoms include the following:
Itching of the skin followed by hives, a rash of raised, reddish bumps or wheals
Swelling of the lips and mouth
Belly cramps
Nausea
Vomiting
Diarrhea
Other symptoms may include the following:
Itching and watering in the eyes
Runny or stuffy nose
Symptoms of a more severe reaction could include the following:
Shortness of breath or difficulty breathing
Tightness in the chest
Feeling of tightness or choking in the throat
Rapid or irregular heart beat
Feeling dizzy or light-headed
Losing consciousness
A severe allergic reaction can be life threatening. This severe reaction is referred to as anaphylaxis, or anaphylactic shock.
The dizziness, light-headedness, and loss of consciousness are due to dangerously low blood pressure, called "shock."
An anaphylactic reaction can begin suddenly, or it may develop gradually with itchiness and swelling of the skin and throat and then progress to a severe reaction over a few hours.
Most people get such a reaction immediately after eating the food, but in a few unusual cases the reaction occurs only after exercising following the ingestion of the food.
Severe reactions are most often seen with allergies to nuts, fish, and shellfish, although allergy to any food can cause anaphylaxis.
People with asthma, childhood allergies, eczema, or prior severe food allergies are especially at risk for having an anaphylactic reaction.
MSG reaction can be mistaken for an allergic reaction.
Symptoms of MSG reaction include the following:
Burning sensation at the back of the neck and radiating down the arms and chest
Tingling and numbness in the same areas
Headache
Nausea
Occasionally, difficulty breathing, especially in those with poorly controlled asthma
A few people experience seizures, irregular heartbeats, and anaphylaxis after the use of MSG.
Contrary to popular beliefs, MSG has no relation to Alzheimer disease, Huntington chorea, or other chronic illnesses.
Exams and Tests
Generally a food allergy is identified by signs and symptoms. Medical professionals are trained to recognize hives, swelling patterns, rashes, and other symptoms associated with allergic reactions.
You will be asked questions about your medical history and possible triggers of the reaction.
Blood tests and other tests are needed only under very unusual circumstances, such as anaphylaxis.
Some people can pinpoint which food caused the allergic reaction, especially if the reaction occurs within minutes of consuming a particular food. Many others will need to see an allergist for special testing to determine the exact food that is responsible.
Self-Care at Home
For localized hives or other mild skin reactions
Take cool showers or apply cool compresses.
Wear light clothing that doesn't irritate your skin.
Take it easy. Keep your activity level low.
To relieve the itching, apply calamine lotion or take over-the-counter antihistamines, such as diphenhydramine (Benadryl) or chlorpheniramine maleate (Chlor-Trimeton).
For all other reactions, especially severe reactions, self-treatment is not recommended. Have a companion drive you to the hospital emergency department, or call 911. Here's what you can do while waiting for the ambulance:
Try to stay calm.
If you can identify the cause of the reaction, prevent further exposure.
Take an antihistamine (1-2 tablets or capsules of diphenhydramine [Benadryl]) if you can swallow without difficulty.
If you are wheezing or having difficulty breathing, use an inhaled bronchodilator such as albuterol (Proventil) or epinephrine (Primatene Mist) if one is available. These inhaled medications dilate the airway.
If you are feeling light-headed or faint, lie down and raise your legs higher than your head to help blood flow to your brain.
If you have been given an epinephrine kit, inject yourself as you have been instructed. The kit provides a premeasured dose of epinephrine, a prescription drug that rapidly reverses the most serious symptoms (see Follow-up).
Bystanders should administer CPR to a person who becomes unconscious and stops breathing or does not have a pulse.
If at all possible, you or your companion should be prepared to tell medical personnel what medications you have taken that day, what you usually take, and your allergy history.

Back