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Tell me about Allergies!

Food Allergies

Not all reactions to foods are actually food allergies!

There are two types of true (immunologically-mediated) food allergies. 

  1. The typical true immediate food allergy is one such as peanuts or shrimp.  A reaction occurs nearly immediately after eating the food, happens even with a tiny or invisible amount of the food, and can be severe or life-threatening.  This type of reaction is caused by IgE, the same part of the immune system that causes hayfever symptoms, dust allergy, etc. 
  2. The second immunologically-mediated food allergy has symptoms that usually develop over several hours after eating the food.  The amount of symptoms may be related to how much of the food is eaten (that is, a person may tolerate a small amount of a particular food without having problems, but develop symptoms if more is eaten).  Sometimes these are caused by other immunoglobulins than IgE, and sometimes by another part of the immune system altogether.

Sometimes a person's history makes it clear that a particular food (or foods) is responsible.  If not, a blood test checking for food-specific IgE can be used to guide the diagnosis.  Or skin testing as done for inhalant (environmental) allergens may suggest a food allergy.  A skin test that is negative is pretty reliable in saying that person is not allergic to that food, but a positive skin test doesn't necessarily mean that food is causing clinical symptoms.

Sometimes being sure about a diagnosis of food allergy requires a double-blinded placebo controlled food challenge (DBPCFC).  This is carried out in a hospital or doctors' office.  Capsules are prepared by the pharmacy that contain either a small amount of the food in question, or a similar amount of an inert (non-reacting) substance.  Neither the doctor nor the patient know which capsule is which.  One at a time, the patient takes a capsule.  Vital signs (pulse, blood pressure, breathing rate, etc) are monitored, and the patient reports any symptoms s/he feels.  If the food capsule causes changes and the inert one doesn't, a diagnosis of allergy to that food is made.  The usual treatment for this is strict avoidance of this food, and carrying self-injectable epinephrine (EpiPen, TwinJect) to use in case of accidental exposure. 

The other broad category of reactions to foods is food intolerance.  These are food reactions that aren't caused by the immune system at all, so they aren't allergies.  They may be enzyme deficiencies, direct effects of the food on histamine, or even food poisoning.

For example, you may know someone who is lactose intolerant.  This isn't a true allergy-  it is a deficiency of the enzyme needed to digest the proteins in milk.  Sometimes lactose intolerant people can tolerate other dairy products, like cheese or yogurt. 

There are some foods that contain a naturally-occurring chemical similar to the body's main chemical messanger of allergy, histamine- or chemicals that can cause the body to release histamine directly without any participation by the immune system.  These can cause symptoms in very sensitive individuals.  Foods that may do this include strawberries and tomatoes.

Eating certain spoiled oily fish can produce allergy-like symptoms- but it's really a type of food poisoning called scombroid.  The tricky thing about this type of poisoning is that the fish looks, smells and tastes fine.  Inadequate refrigeration during storage permits the development of histamine-like chemicals.  When the food is eaten, the person develops skin flushing, cramps, nausea, diarrhea, possibly even wheezing.  Often the only clue that it's not a true allergic reaction is that several people who were dining together develop similar symptoms. 

Two other types of reactions to food are Oral Allergy Syndrome and Celiac Disease.  

Here's information about how food allergies are treated now, and how they may be treated in the future.