Definition of food allergy
An allergic reaction to a food can be described as an inappropriate reaction by the body's immune system to the ingestion of a food that in the majority of individuals causes no adverse effects.
An allergy is therefore a harmful immune response to an antigen that is not itself intrinsically harmful.
Food allergy
Notably common during the first two years of life, when the gut barrier is immature
among infants and toddlers
food reactions 6% to 8%
cow’s milk 2% to 2.5%
eggs 1.7% to 2%
peanuts 0.6%
tree nuts 0.6%
by age 10, many children outgrow food allergy, especially to egg or milk
Food allergy
20% to 25% of American public reports being allergic to one or another food
careful studies, with oral challenge, demonstrate actual food allergy in 2% to 2.5% of the population
food allergy is the leading cause of anaphylaxis requiring accident and emergency visits - with twice the incidence, and three times the mortality, of anaphylactic reaction to bee sting
100 to 125 persons die of food reactions each year
Food allergens
food allergens are glycoproteins
egg glycoproteins: ovomucoid, ovalbumin, ovotransferrin, lysozyme
milk casein glycoproteins
peanut glycoproteins: vicilin, conglutin, glycinin
a given food may contain 10 to 30 glycoproteins
food allergens have some resemblance to pollen proteins
broad-spectrum of syndromes include hives “the classic food reaction”, unexplained wheezing or eczema, gastric complaints, and oral allergy symptoms
milk-allergy has been associated with nausea, vomiting, diarrhoea, bloating, early satiety, reflux, oesophagitis, gastritis, malabsoption, abdominal pain, and bloody stool
“oral allergy syndrome” include itching of the mouth, and swelling of the tongues and lips - associated with eating FRESH apple, carrots, hazelnut, bananas, melons
Immunobiology
Antibodies recognize antigens as
linear epitope: an amino acids sequence
conformational epitope: amino acids sequence with a specific 3-dimensional shape. The 3D-shape may alter absorption or be denatured by heat
Mechanisms of food allergy
Allergic reactions to foods vary in severity and can be potentially fatal.
The immune system does not recognize as safe a protein component of the food to which the individual is sensitive (such as some proteins in peanuts)- called the allergen.
The immune system then typically produces immunoglobulin E (IgE) antibodies to the allergen, which trigger other cells to release substances that cause inflammation.
The immune system is tilted toward the production of T helper cells of the Th2 subtype.
These release interleukin 4 (IL-4) and interleukin 13 (IL-13) on the B cell causing it to synthesise IgE antibodies.
Immediate (type I) hypersensitivity
When these cell-bound antibodies encounter allergens, the mast cells discharge their granules releasing histamine and leukotrienes.
This causes local anaphylaxis: swelling, redness, and itching.
Some allergens including peanut proteins can precipitate such a massive IgE-mediated response that a life-threatening collapse of the circulatory and respiratory systems may occur. Immediate (type I) hypersensitivity
Immediate (type I) hypersensitivity
Immediate or type I hypersensitive reactions occur quickly after exposure to the allergen.
They are usually mediated by antibodies of the IgE class.
The constant region of IgE antibodies has a binding site for a receptor present on the surface of basophils and their tissue equivalent - the mast cell.
Immunobiology
gastrointestinal manifestations of food allergy tend to be cell mediated (T-cells, monocytes and macrophages)
Non-gastrointestinal manifestations tend to be IgE mediated
Investigation
History not only helps to pinpoint the suspected food allergen, but also to suggest whether or not the disorder is IgE-mediated
Endoscopy, the next step in evaluating non-IgE food reactions, may confirm the diagnosis histopathologically
Skin testing is the next step in evaluating suspected IgE-related disorders. A positive skin test does not confirm the presence of allergy. Only 40% of patients with a positive skin test to a food allergen turnout to be reactive when the food is ingested. The other 60% requires no further evaluation.
Oral challenge
For patients with positive skin test or endoscopic findings
unblinded oral challenge - eat small amount of the food under clinical observation, if positive then perform
single-blinded oral challenge
to distinguish patient with true food allergy
various strategies to disguise adequate quantities (10 grams) of the food being tested - incorporate in pancakes, canned tuna fish, or medicine capsules
Food allergy
When food allergy is confirmed, it almost always proves to be restricted to 1 or 2 foods
young children: milk, peanut, tree nuts, soy, and wheat account for about 90% of cases
adolescents and adults: peanut, fish, shellfish, and tree nuts accounts for about 85%
Diagnostic Tests
3 reasons for performing diagnostic tests:
. educate patient to avoid allergen
. addressing risk of anaphylaxis
. discuss prospects for specific therapy
Avoiding allergen
simply avoiding allergic food is not enough
must educate patient about specific allergen
caseins, the chief milk-curd proteins, are sometimes added to food to preserve a chunky consistency
inadvertent contamination in manufacturing, cooking, or serving process: peanut oil on cooking or serving utensils, spaghetti sauce with peanut butter as an ingredient
inadequate (or ignored) food labelling
Risks of anaphylaxis
in general, anaphylaxis is unpredictable
may be life-threatening
1/3 may be biphasic (may recur 90 min after initial response to epinephrine, then become unresponsive to repeated treatment)
protracted anaphylaxis occur in about 1/4 of anaphylactic episodes
Individuals at risks for anaphylaxis
patients whose food reactions have had respiratory features such as difficulty breathing or throat tightness
patients who are asthmatic
patients allergic to peanut, fish, shellfish, or tree nuts - the allergies that patients tend not to outgrow
Self injectable epinephrine
Patients judged to have a high risk of anaphylaxis should be equipped with self-injectable epinephrine
the device should be used whenever the patient believes a food reaction is beginning
in view of the threat of a biphasic reaction, patient should be educated to go to a hospital accident and emergency ward immediately afterward and be prepared to stay there for at least 4 hours
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