Reactions to foods can contribute to any number of symptoms in various body systems. There are a number of different types of reactions that can occur depending on which part of the immune system is involved.

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Type I Allergic Reactions

A Type I reaction can be life-threatening. A reaction may consist of mild to severe swelling of the lips, mouth, throat and face, vomiting, wheezing from airway swelling and hives (urticaria). This type of reaction is what is known as a "true allergy". This type of allergy is diagnosed by doing a scratch test for food or by doing a blood test, both of which show whether or not there are IgE antibodies present. This is the type of allergy for which an EpiPen is prescribed.

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Type II Allergic Reactions

Type II allergies are antibody dependent reactions to lectins binding to ABO markers on cells, initiating an immune reaction through complement proteins. 

 

Type III Delayed Hypersensitivity Reactions 

In a Type III allergy, the most common types of reaction include rashes, diarrhea or constipation, breathing problems and fatigue. This type of reaction can take up to 72 hours to manifest, which can often make it very difficult for a patient to figure out what they are reacting to! The Type III allergy is diagnosed by doing a blood test for IgG antibodies. This is a common type of testing used to guide elimination diets like the one shown on this resource page. This type of test can yield false positives, particularly when the digestion is already poor. 

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Type IV Delayed Sensitivity 

Type 4 Delayed Sensitivity T-Cell mediated allergy in skin and soft tissues. This type of reaction takes 2-3 days to develop. Contact dermatitis is one form of this type of allergy and can be initiated by contact with poison ivy or latex.

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Food Intolerances

Food intolerances may exist as a result of enzyme deficiencies in the body due to inborn or acquired errors of metabolism. These can mimic true reactions and delayed hypersensitivity reactions. Examples are lactose intolerance, fructose intolerance, alcohol intolerance, histamine intolerance, salicylate intolerance (see the Feingold diet for salicylate intolerance modifications). 

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Bad Food Reactions

People can also react to biological contaminants or poisons present in a foodstuff. An example would be scromboid toxicity from fish. 

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Oral Allergy Syndrome

Oral Allergy Syndrome consists of reactions that are localized to the mouth and throat, occurring within minutes of ingestion of uncooked fruits and vegetables such as apple, peach, celery, tomato and cherry. There are often cross reactions to particular pollens that share antigenic similarity with the foods. See this handout for more information.