Peanut allergy is a type of food allergy to peanuts. It is different from nut allergies. Physical symptoms of allergic reaction can include itchiness, urticaria, swelling, eczema, sneezing, asthma, abdominal pain, drop in blood pressure, diarrhea, and cardiac arrest. Anaphylaxis may occur.
It is due to a type I hypersensitivity reaction of the immune system in susceptible individuals. The allergy is recognized "as one of the most severe food allergies due to its prevalence, persistency, and potential severity of allergic reaction."
Prevention may be partly achieved through early introduction of peanuts to the diets of pregnant women and babies. The principal treatment for anaphylaxis is epinephrine as an injection.
In the United States, peanut allergy is present in 0.6% of the population. It is a common cause of food-related fatal and near-fatal allergic reactions.
Video Peanut allergy
Signs and symptoms
Symptoms of peanut allergy are related to the action of Immunoglobulin E (IgE) and other anaphylatoxins which act to release histamine and other mediator substances from mast cells (degranulation). In addition to other effects, histamine induces vasodilation of arterioles and constriction of bronchioles in the lungs, also known as bronchospasm.
Symptoms can include mild itchiness, urticaria, angioedema, facial swelling, rhinitis, vomiting, diarrhea, acute abdominal pain, exacerbation of atopic eczema, asthma, and cardiac arrest. Anaphylaxis may occur.
Maps Peanut allergy
Cause
The cause of peanut allergy is unclear and at least 11 peanut allergens have been described. The condition is associated with several specific proteins categorized according to four common food allergy superfamilies: Cupin (Ara h 1), Prolamin (Ara h 2, 6, 7, 9), Profilim (Ara h 5), and Bet v-1-related proteins (Ara h 8). Among these peanut allergens, Ara h 1, Ara h 2, Ara h 3 and Ara h 6 are considered to be major allergens which means that they trigger an immunological response in more than 50% of the allergic population. These peanut allergens mediate an immune response via release of Immunoglobulin E (IgE) antibody as part of the allergic reaction.
Some of the peanut allergens can undergo enzymatic and non-enzymatic modifications which makes them more likely to bind to ligands on antigen-presenting cells. Ara h 1 can undergo glycosylation modifications which have been shown to induce immunomodulatory responses; it stimulates lectin receptors MR and DC-SIGN on dendritic cells which further propagate cytokines and bias the immune system towards a Th2 type response. Peanut proteins that undergo non-enzymatic changes through Maillard reactions when cooked or exposed to room temperature have an increase in AGE modifications on their structure. These changes have been shown to stimulate RAGE receptors and SR-AI/II on dendritic cells and thus lead to an increase in IL-4 and IL-5-releasing Th2 cells.
Peanut allergies are uncommon in children of undeveloped countries where peanut products have been used to relieve malnutrition. The hygiene hypothesis proposes that the relatively low incidence of childhood peanut allergies in undeveloped countries is a result of exposure to diverse food sources early in life, increasing immune capability, whereas food selection by children in developed countries is more limited, reducing immune capability. A possibility of cross-reaction to soy was dismissed by an analysis finding no linkage to consumption of soy protein, and indicated that appearance of any linkage is likely due to preference to using soy milk among families with known milk allergies.
Timing of exposure
When infants consume peanut proteins while 4 to 11 months old, the risk of developing peanut allergy before the age of 5 years decreases by 11-25%, specifically in children with higher allergy risk via their parents with peanut allergy. From these results, the American Academy of Pediatrics rescinded their recommendation to delay exposure to peanuts in children, also stating there is no reason to avoid peanuts during pregnancy or breastfeeding.
Diet during pregnancy
There is conflicting evidence on whether maternal diet during pregnancy has any effect on development of allergies due to a lack of good studies. A 2010 systematic review of clinical research indicated that there is insufficient evidence for whether maternal peanut exposure, or early consumption of peanuts by children, affects sensitivity for peanut allergy.
Routes of exposure
While the most obvious route for an allergic exposure is unintentional ingestion, some reactions are possible through external exposure. Peanut allergies are much more common in infants who had oozing and crusted skin rashes as infants. Sensitive children may react via ingestion, inhalation, or skin contact to peanut allergens which have persistence in the environment, possibly lasting over months.
Airborne particles in a farm- or factory-scale shelling or crushing environment, or from cooking, can produce respiratory effects in exposed allergic individuals. Empirical testing has discredited some reports of this type and shown some to be exaggerated. Residue on surfaces has been known to cause minor skin rashes, though not anaphylaxis. In The Peanut Allergy Answer Book, Harvard pediatrician Michael Young characterized this secondary contact risk to allergic individuals as rare and limited to minor symptoms. Some reactions have been noted to be psychogenic in nature, the result of conditioning, and belief rather than a true chemical reaction. Blinded, placebo-controlled studies were unable to produce any reactions using the odor of peanut butter or its mere proximity.
Pathophysiology
The allergy arises due to dendritic cells recognizing peanut allergens as foreign pathogens. They present the antigens on MHC class II receptors and these antigens are then recognized by cell receptors on T cells. The contact along with IL-4 induces their differentiation into CD4+ Th2 cells. These Th2 cells proliferate and release pro-inflammatory cytokines such as IL-4, IL-5 and IL-13 which can be bound to on undifferentiated B cells or B cells of the IgM subtype. This binding causes their differentiation into IgE which can then be bound onto Fc?RI on mast cells, eosinophils and basophils. The binding causes a degranulation of the aforementioned cells which release potent cytokines and chemokines, thus triggering an inflammation and causing the symptoms characteristic of allergy.
Diagnosis
Diagnosis of food allergies, including peanut allergy, begins with a medical history and physical examination. National Institute of Allergy and Infectious Diseases guidelines recommend that parent and patient reports of food allergy be confirmed by a doctor because "multiple studies demonstrate 50% to 90% of presumed food allergies are not allergies."
Skin prick testing
Skin prick tests can be used to confirm specific food allergies. Skin prick tests are designed to identify specific IgE bound to cutaneous mast cells. During the test, a glycerinated allergen extract drop is placed on the patient's skin. The patient's skin is then pricked through the drop. This procedure is repeated with two controls: a histamine drop designed to elicit an allergic response, and a saline drop designed to elicit no allergic response. The wheal that develops from the glycerinated extract drop is compared against the saline control. A positive allergic test is one in which the extract wheal is 3mm larger than the saline wheal. A positive skin prick test is about 50% accurate, so a positive skin prick test alone is not diagnostic of food allergies.
Oral food challenge
The "gold standard" of diagnostic tests is a double-blind placebo-controlled oral food challenge. At least two weeks prior to an oral food challenge, the person is placed on an elimination diet where the suspected allergen is avoided. During the oral food challenge, they are administered a full age-appropriate serving of a suspected allergen in escalating size increments. They are continuously monitored for allergic reaction during the test, and the challenge is stopped and treatment administered at the first objective sign of allergic reaction.
Oral food challenges pose risks. In a study of 584 oral food challenges administered to 382 patients, 48% (253) of challenges resulted in allergic reactions. 28% (72) of these challenges resulted in "severe" reactions, which were defined by the study as a patient having: lower respiratory symptoms; cardiovascular symptoms; or any four other, more minor, symptoms. Double-blind placebo-controlled oral food challenges are also time consuming and require close medical supervision. Because of these drawbacks to the double-blind placebo-controlled oral food challenge, open food challenges are the most commonly used form of food challenge. Open food challenges are those in which a patient is fed an age-appropriate serving of a suspected food allergen in its natural form. The observation of objective symptoms resulting from ingestion of the food, such as vomiting or wheezing, is considered diagnostic of food allergy if the symptoms correlate with findings from the patient's medical history and laboratory testing such as the skin prick test.
Prevention
Peanut allergy may be preventable by feeding babies who are at high risk foods that contain peanuts when they are as young as four to six months of age.
Treatment
Currently there is no cure for allergic reactions to peanuts other than strict avoidance of peanuts and peanut-containing foods. Extra care needed for food consumed at or purchased from restaurants. The principal treatment for anaphylaxis is epinephrine as an injection.
Prognosis
Peanut allergies tend to resolve in childhood less often than allergies to soy, milk, egg, and wheat. Accordingly, re-evaluation of peanut allergy is recommended on a yearly basis for young children with favorable previous test results, and every few years or longer for older children and adults.
Epidemiology
The percentage of people with peanut allergies is 0.6% in the United States. In a 2008 study, self-reported incidence of peanut allergy was estimated to affect 1.4% of the population of the United States, triple the 0.4-0.6% rate found in a 1997 study. In England, an estimated 4,000 people are newly diagnosed with peanut allergy every year; 25,700 having been diagnosed with peanut allergy at some point in their lives.
Peanut allergy is one of the most dangerous food allergies, and one of the least likely to be outgrown. In Western countries, the incidence of peanut allergy is between 1-3%. There has been a sudden increase in number of cases in the early 21st century.
It is one of the most common causes of food-related deaths. A meta-analysis found that death due to overall food-induced anaphylaxis was 1.8 per million person-years in people having food allergies, with peanut as the most common allergen. However, there are opinions that the measures taken in response to the threat may be an over-reaction out of proportion to the level of danger. Media sensationalism has been blamed for anxiety outweighing reality.
Frequency among adults and children is similar--around 1%--but one study showed self-reports of peanut allergy are on the rise in children in the United States. The number of young children self-reporting the allergy doubled between 1997 and 2002. Studies have found that self-reported rates of food allergies is higher than clinically-observed rates of food allergies. The rates in self-reported incidence of the allergy, previously thought to be rare, may not be correlated with medical data confirming the self-reported incidence.
Society and culture
The high severity of peanut allergy reactions, as well as the increasing prevalence of peanut allergy in the Western world have led to widespread public attention. However, the perceived prevalence of food allergies in the public view is substantially higher than the actual prevalence of food allergies.
Because food allergy awareness has increased there are impacts on the quality of life for children, their parents and their immediate caregivers. In the United States, the Food Allergen Labeling and Consumer Protection Act of 2004 causes people to be reminded of allergy problems every time they handle a food package, and restaurants have added allergen warnings to menus. The Culinary Institute of America, a premier school for chef training, has courses in allergen-free cooking and a separate teaching kitchen. School systems have protocols about what foods can be brought into the school. Despite all these precautions, people with serious allergies are aware that accidental exposure can still easily occur at other peoples' houses, at school or in restaurants. Food fear has a significant impact on quality of life. Finally, for children with allergies, their quality of life is also affected by actions of their peers. There is an increased occurrence of bullying, which can include threats or acts of deliberately being touched with foods they need to avoid, also having their allergen-free food deliberately contaminated.
Research
Immunotherapy involves attempts to reduce or eliminate allergic sensitivity by repeated exposure. This active research concept involves swallowing small amounts of peanuts, holding a peanut product under the tongue - sublingual immunotherapy - skin patches or injections. None of these are considered ready for use in people outside of carefully conducted trials. In those with mild peanut allergies, gradually eating more and more peanuts resulted in at least some short-term benefits. Due to the amount of evidence being small and the high rate of adverse effects, this is not currently recommended as treatment. Sublingual immunotherapy involves putting gradually increasing doses of an allergy extract under a person's tongue. The extract is then either spat or swallowed. It is not currently recommended as treatment; however, it is being studied. Epicutaneous immunotherapy involves giving the allergen through a patch. Trials are ongoing.
See also
- Allergy (has diagrams showing involvement of different types of white blood cells)
- Food allergy (has images of hives, skin prick test and patch test)
- List of allergens (food and non-food)
- Tree nut allergy (can be cross-reactive to peanut allergy)
References
Source of article : Wikipedia
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