Food allergy occurs when the body's immune system overreacts to food proteins which are eaten.  From shortly after birth, the body must learn which proteins are good (i.e., milk protein) and which are bad (i.e., germs).  This complex process involves training of the body's white blood cells and specifically T-lymphocytes.  These T-lymphocytes are trained to recognize good proteins (i.e., pizza) and to raise the alarm when the body is invaded by germs.  When this process goes awry, a food allergy may develop.  There are 3 major types of food allergy: IgE mediated, cell mediated, and mixed type.  In some instances, an IgE mediated food allergy can cause a life-threatening anaphylactic reaction.  Symptoms of food allergy may include: breathing problems (anaphylactic type), swelling, wheezing, tingling of the tongue and mouth, vomiting, diarrhea, or skin rashes.  Treatment for food allergy mostly involves avoidance of the offending food types. 

What are the types of food allergy?

IgE mediated

This type is caused by a complex series of events of the immune system involving T-cells, B-cells, antibodies, mast cells and basophils.  In peanut allergy, the process is as follows:

1. A peanut is eaten.
2. The peanut is digested and peanut proteins enter the blood.
3. T-cells discover the peanut proteins, recognize them as bad, and send signals to B-cells (another type of white blood cell).
4. B-cells release IgE (an immunoglobulin or antibody) that recognizes peanut proteins.
5. The IgE attaches to mast cells and basophils (special cell types which release allergic reaction chemicals).
6. Another peanut is eaten.
7. The peanut is digested and peanut proteins enter the blood.
8. The peanut proteins attach to IgE which is attached to mast cells and basophils.
9. The mast cells and basophils release chemicals (i.e., histamine) which causes all the symptoms of an allergic reaction.

 Cell mediated

This type is caused T-lymphocytes.  T-lymphocytes, sometimes called T-cells, are a type of white blood cell produced in the bone marrow, and which "go to school" in the thymus.  In peanut allergy, the process is as follows:

1. A peanut is eaten.
2. The peanut is digested and peanut proteins enter the blood.
3. T-cells discover the peanut proteins, recognize them as bad, and release chemicals.
4. These chemicals cause inflammation and induce other white blood cells to join the inflammatory process.

Mixed type

This type involves both the IgE mediated and cell mediated processes described above.

 How common is food protein allergy?

• Up to 8% of children less than 3 years of age have some type of food protein allergy.
• About 2.5% of infants have milk protein allergy in the first year of life.  Over 80% of these infants "outgrow" milk protein allergy by 5 years of age.
• Almost 40% of children with eczema have some type of food allergy. 

What are the symptoms of food allergy?

IgE mediated (anaphylaxis)

• Low blood pressure
• Fast heart rate
• Breathing difficulty
• Swelling of the face, tongue and throat

IgE mediated (non-anaphylactic)

• Skin rashes or welts
• Swelling
• Vomiting
• Cramping
• Tingling of the tongue or mouth
• Runny nose
• Nausea
• Abdominal pain
• Diarrhea (may occur several hours after exposure to food) 

Cell mediated

• Weight loss
• Vomiting
• Loss of appetite
• Diarrhea
• Eosinophilic esophagitis
• Blood in stools
• Bloating 

Mixed type

Any combination of the symptoms above.

How is food allergy diagnosed?

• History and physical
• Food exposure and avoidance trials (i.e., trial and error)
• Allergy testing (i.e., skin prick, skin patch, blood tests)
• Endoscopy with biopsies

How is food allergy treated?

• Anaphylactic reactions are a medical emergency (call 911!)
• The treatment depends on the allergy type and the severity.
• Avoidance of the food type (if known).

 What are the most common food allergies in children?

• Milk
• Eggs
• Soy
• Wheat
• Shellfish
• Nuts 

What are some special types of food allergy?

• Celiac disease
• Eosinophilic esophagitis

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