Given that this text refers to the issue of allergies, I recommend reading it along with the introductory text dedicated to allergy, which defines the terms such as allergy, atopy, allergen and allergic reactions.
Food allergy is a condition where an allergic reaction is triggered by eating certain food. It is relatively frequent, especially in the developed countries.
The allergic reaction is caused by an allergen present in ingested food. Such allergen is individual. Some people are allergic to substances that do absolutely no harm to other people and some people are not allergic at all. However, there are foods that tend to cause allergic reactions more often such as exotic imported food, tropical fruits, sea fish, etc. More frequently reported food allergies also include allergy to nuts, berries, cereals, milk and some types of spices.
The manifestations of food allergy may be extremely variable and they include virtually any allergic symptoms such as asthma (wheezing and whistling during exhalation, shortness of breath, dry cough), urticaria (itchy red rash), allergic conjunctivitis (redness of the eyes, burning eyes swelling of the eyelids), atopic eczema and sometimes even the dangerous anaphylactic shock. Details can be found in the relevant texts.
Food allergies also cause gastrointestinal symptoms as it is the way of the allergen entrance into the body. The possible symptoms include diarrhea, abdominal pain, abdominal cramps and special esophageal disease known as eosinophilic esophagitis.
It is important to find out the food the patient is allergic to. An allergological examination including the skin tests is advisable.
The main way of prevention is avoiding the food that triggers the allergic reaction.
It depends on the severity of symptoms. Mild symptoms require no treatment; annoying allergic manifestation may be treated by various anti-allergic drugs, for example antihistaminic agents or corticosteroids. Urgent and aggressive therapy is needed in case of the anaphylactic shock (see relevant article).