Food Allergy
last authored: Oct 2009, David LaPierre
last reviewed:
Introduction
Food allergy is a harmful immune response to proteins and other molecules found in food. Allergy can form against any food, though there are a number of common and important allergens. These include:
- milk (most common)
- peanut and tree nuts (most severe)
- egg
- soy
- sesame
- seafood
- wheat
In North America, there is a prevalence of 5% in children and 2% of adults in North America. However, The public vastly overestimates rates of allergy, and only 1/4 presumed reactions confirmed with testing.
There also seem to be an increase in food allergy rates. (JACI 2002 110:784-9). Peanut sensitization increased 3x over 7 years from 1.1% to 3.3%. This increase seems to be only in the developed world (hygiene hypothesis).
The Case of...
A 5 year-old girl with no previous health issues
- child became quite upset, with sneezing and runny nose
Causes and Risk Factors
The most common cause of food allergy is IgE-mediated, as described below.
Allergens are usually water-soluble glycoproteins with three-dimensional structure. Epitopes can be sequential epitope or conformational. Conformational epitopes can disappear when a protein is heated or denatured, reducing or eliminating the allergenic effect.
Highly stable
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Partially unstable
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Highly unstable
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There is a range of other types of allergic response as well, including:
- cell-mediated (celiac disease, food protein-induced enterocolitis)
- mixed IgE and cell-mediated: atopic dermatis, asthma, allergic eosinophilic esophagitis or gastroenteritis
Pathophysiology
Allergy begins when IgE forms against allergens and begins to circulate. Mast cells, the principal effectors of the allergic response bind IgE on their cell surface, resulting in sensitized cells. They are richest in the skin, gut, eyes, and other areas.
Once the allergen is encountered by IgE on the cell surface, mast cells release a series of waves of responses.
1) Granules are released immediately and are packed with inflammatory mediators, notably histamine, TNF-a, proteases, heparin
- leads to increased vascular permeability and endothelial activation
2) over minutes: lipids (prostaglandins, leukotrienes)
3) over hours: cytokines (IL-4, IL-13) to recruit inflammatory cells, ie eosinophils.
IgE has a short half-life, perhaps 2 days, but is present for decades or a life-time.
Signs and Symptoms
Diagnosis is made by two essential elements:
- history of IgE-mediated hyersensitivity
- evidence of food-specific IgE to historically implicated foods
- history
- physical exam
History
The following are symptoms of the allergic response, progressing to anaphylaxis if severe
- flushing
- pruritus
- uriticaria: often with central clearing if severe
- conjunctival injection
- wheezing
- rhinorrhea
- abdominal pain
- diarrhea
Skin and gut symptoms are almost universally present. Acute rhinitis is the most common presenting complaint in children.
As the allergic response progresses, anaphylaxis, or severe, multi-system response, can occur.
- impending doom
- laryngeal edema
- angioedema
- bronchospasm
- cardiac arrhythmias (late)
- hypotension
- vascular instability
- uterine contractions
When questioning patients about the possibility of food allergy, ask them:
- likely responsible foods
- allergy severity (timing, quantity ingested, severity of response, treatment required?)
- co-morbid atopic conditions (dermatitis, eczema)
- family history
Risk factors of severe anaphylaxis include:
- severe asthma
- beta blockers
Physical Exam
Eczema
Investigations
- lab investigations
- diagnostic imaging
Lab Investigations
Skin test
- non longer positive or negative
- now viewed as probability of reactivity, based on quantity of IgE or wheal size
measured at 10 minutes
excellent negative predictive value (>95%)
commercial extracts have stable epitopes, while testing for unstable epitopes found in fruits and vegetables require crude food extracts to be tested.
Quantitative Food-Specific IgE
Food-specific IgE confirms sensitization, but may not necessarily be clinically important.
The numeric value of the IgE is what is important, and differs according to allergen
Poor negative predictive value for many allergens, and many false positives as well.
Level does not predict severity.
Diagnostic Imaging
Endoscopy, to assess and biopsy the esophagus, stomach, and small intestine, can be performed to assess for celiac disease, enterocolitis, or eosinophilic esophagitis or gastroenteritis
Differential Diagnosis
There are many other food reactions:
- lactose intolerance
- additive intolerance
- food poisoning (bacteria, toxins)
Allergy or hypersensitivity are discriminated by...
Treatments
Epinephrine is first-line if anaphylaxis is suspected. As it stablizes mast cells, it should be given as quickly as possible if severe allergic response is considered.
Oral immunotherapy is an emerging therapy, and relies on small amounts of oral challenge given daily and increased over a series of weeks. It appears to raise the threashold of reactivity in 80% of selected cases of milk, egg, and peanut allergy, though children often become symptomatic with the challenge and 1/600 children required epinephrine. The long-term impact is not known.
Prevention
Food avoidance during pregnancy or breastfeeding appears to be of no benefit.
The benefit of delayed introduction into a child's diet is unknown, but it is a good idea to wait until a few years of age to better be able to evaluate response.
Consequences and Course
Food allergies are often outgrown by children, who unfortunately are at higher risk for further immunological problems, described by the allergic march: food allergy, progressing to atopic dermatitis, leading to environmental allergy.
Resources and References
Food Allergy and Anaphylaxis Network
Topic Development
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