When the body's immune system reacts abnormally to something a person eats or drinks, it's known as a food allergy.
Food allergies may affect as many as 220 to 520 million people worldwide, with the majority of those sufferers being children. It is estimated that more than 12 million Americans have diagnosed food allergies.
A food allergy may affect the skin, the gastrointestinal tract, or the respiratory or cardiovascular systems. Many types of foods can be allergens, but certain foods are much more likely than others to trigger an allergic reaction.
In fact, a mere eight foods are responsible for 90 percent of food allergies in North America: cow's milk, eggs, peanuts, fish, shellfish, tree nuts (such as cashews or walnuts), wheat, and soy.

Symptoms of Food Allergies

Symptoms of food allergies may range from mild to severe and they may come on suddenly or develop over several hours.
Because a person's immune system may react to a very small amount of the allergen, food allergies are particularly dangerous and potentially life threatening—especially if breathing is affected. Because of this, people with asthma are at an increased risk for a fatal allergic reaction to food.
Mild symptoms related to a food allergy may include:
  • sneezing
  • stuffy or runny nose
  • itchy, watery eyes
  • swelling
  • rash
  • stomach cramps
  • diarrhea
Severe symptoms of an allergic reaction to food are:
  • difficulty breathing, including wheezing
  • swelling of the lips, tongue or throat
  • hives (an itchy, blotchy and raised rash
  • dizziness or faintness
  • nausea or vomiting

Milk Allergies

A milk allergy (not to be confused with lactose intolerance) is a reaction to whey or casein, the proteins found in cow's milk. Milk allergies have been studied extensively—more than any other food allergy.
The bad news is that children with milk allergies are much more likely to develop allergic reactions to other foods including eggs (58 percent), soy (47 percent), and peanuts (34 percent), according to one study.
Most children with milk allergies also develop one or more other atopic diseases such as asthma, allergic rhinitis, or eczema.

Egg Allergies

Egg allergies occur most often in children and usually resolve at a very young age (most by age five), although some people may remain allergic to eggs for their entire lives.
A person may be allergic to a certain protein in either the yolk or the egg whites (therefore, a person with an allergy to the egg yolk may be able to tolerate egg whites and vice versa), although some are allergic to both.

Peanut Allergies

Children with peanut allergies rarely grow out of their sensitivity to peanuts, so a peanut allergy is usually a lifelong disorder. Because of this, peanut allergies are particularly serious, as accidental exposure can occur at any time during a person's life. Though rare, a peanut allergy may result in anaphylaxis—a severe allergic reaction that can restrict breathing or cause cardiac arrest. Anaphylaxis requires immediate medical attention in the form of a shot of epinephrine (EpiPen). A patient should be watched for several hours after the shot to make sure symptoms do not return.

Other Common Allergies

Less is known about soy and wheat allergies than the more common allergies discussed above,.  Likewise, little is known about fish, shellfish, and tree nut allergies except that, like a peanut allergy, they are generally lifelong disorders.

Diagnosis

Food allergies are usually diagnosed depending on the severity of symptoms.
If a patient's symptoms are mild, a doctor may recommend she keep a food diary to record all of the foods she eats or drinks to pinpoint the culprit. Another way to diagnose a mild food allergy is to remove certain foods from the diet and then slowly reintroduce them to find out if symptoms return.
In the case of more severe allergies, skin or blood tests can identify egg, milk, nut, and shellfish allergies.

Treatments

As with other types of allergies, avoidance is most often the best medicine. A person who suffers from food allergies herself or has a child with a food allergy should be vigilant when purchasing? food at a supermarket or restaurant to make sure there are no traces of the allergen in a food or meal.
Milder symptoms may not require any treatment at all, or a simple over-the-counter antihistamine may resolve the symptoms.
For more serious allergic reactions, a doctor may prescribe steroid medications. Steroids may have serious side effects, however, and shouldn't be used for more than a few days at a time.
There is evidence that feeding a baby with only breast milk for the first four to six months of life lowers the baby’s risk of developing a food allergy in the future. Mothers who are nursing a child who already has a food allergy should avoid eating the offending food, as allergens can be transferred to the baby via breast milk.