An allergy can be described as a malfunction of the immune system, an  exaggerated response to certain substances. His body mistakenly believes  that something he has touched, smelled or ingested is harmful to him  and his body releases massive amounts of chemicals, such as histamine,  to protect itself.

11 million Americans are believed to have food allergies. These allergies are as varied as the foods themselves. Some people suffer from an allergy to one food, others to many. The most common food allergies are generally eggs, milk, peanuts, tree nuts (such as walnuts), fish, shellfish, soybeans, and wheat.
Symptoms of food allergies are varied, ranging from a tingling in the mouth to swelling of the tongue and throat, difficulty breathing, hives, cramps, diarrhea, vomiting and, in some cases, death.
There are ways to help you or your loved one manage your food allergies. Seek the help of an allergist first. Your allergist will perform a patch test to determine the exact cause of your allergic reactions. This will be the guideline you will use as you develop a diet based on your food allergies.
As with other types of allergies, there is no cure for a food allergy. Some children outgrow some food allergies as they age, although allergies to peanuts, fish, shellfish, and tree nuts are often considered lifelong allergies. You or your loved one should simply avoid the foods that cause the allergy. This can be difficult, especially when eating in a restaurant. Depending on the severity of the allergy, even slight cross contamination of food products can cause reactions.
Food labeling is a very important component of avoiding allergy-causing foods. Since 2000, the FDA has presented allergy risk information and labeling requirements to manufacturers. They want manufacturers to change some labels to make them easier to read, using simple language like “milk” on a label instead of “caseinate.”
In the case of a milk or egg allergy, there are alternatives that can be used when cooking or baking. There are many sites online dedicated to providing information, education, and support to people with food allergies.