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Oral allergy syndrome or OAS is a type of food allergy classified by a cluster of allergic reactions in the mouth in response to eating certain (usually fresh) fruits, nuts, and vegetables that typically develops in adult hay fever sufferers.
OAS is perhaps the most common food-related allergy in adults. OAS is not a separate food allergy, but rather represents cross-reactivity between distant remnants of tree or weed pollen still found in certain fruits and vegetables. Therefore, OAS is typically only seen in tree and weed allergic patients, and is usually limited to ingestion of only uncooked fruits or vegetables.
OAS is a Type 1 or IgE-mediated immune response, which is sometimes called a "true allergy". The body's immune system produces IgE antibodies against pollen; in OAS, these antibodies also bind to (or cross-react with) other structurally similar proteins found in botanically related plants.
OAS can occur any time of the year but is most prevalent during the pollen season. Individuals with OAS usually develop symptoms within a few minutes after eating the food.
OAS sufferers may have any of a number of allergic reactions that usually occur very rapidly, within minutes of eating a trigger food. The most common reaction is an itching or burning sensation in the lips, mouth, ear canal, and/or pharynx. Sometimes other reactions can be triggered in the eyes, nose, and skin. Swelling of the lips, tongue, and uvula and a sensation of tightness in the throat may be observed. It can seldom result in anaphylaxis. If a sufferer swallows the food, and the allergen is not destroyed by the stomach acids there is a good chance that there will be a reaction from histamine release later in the gastrointestinal tract. Vomiting, diarrhea, severe indigestion, or cramps may occur. Rarely, OAS may be severe and present as wheezing, vomiting, hives and low blood pressure.
In OAS, the immune system produces antibodies that are capable of binding to both pollen proteins and structurally similar food proteins. Consequently, the same immune system response can trigger allergy symptoms in two different situations: hay fever (in the presence of pollen) and food allergy (in the presence of certain foods). Histamine releases from mast cells located in the oropharynx, gut and skin when IgE binds to the molecule causing local inflammation.
The triggering molecule involved is known as an allergen. Allergens vary in their stability and may or may not survive digestion, storage, heat, cold, cooking or pasteurisation.
Lipid transfer proteins (LTP) are not easily denatured by digestion or cooking and are important triggers of anaphylaxis.
The antibody may react to the linear (amino acid) sequence of the protein or to a conformational epitope. If the response is to the conformational epitope, then the person with OAS may be able to eat the food when it is cooked, but not when it is raw. If the response is to the linear sequence (common in tree pollen/nut allergies), then cooking the food has no effect on its ability to trigger an allergic reaction.
OAS produces symptoms when an affected person eats certain fruits, vegetables and nuts. Some individuals may only show allergy to only one particular food, and others may show an allergic response to many foods.
Individuals with an allergy to tree pollen may develop OAS to a variety of foods. While the tree pollen allergy has been worked out, the grass pollen is not well understood. Furthermore, some individuals have severe reactions to certain fruits and vegetables that do not fall into any particular allergy category. In recent years, it has also become apparent that when tropical foods initiate OAS, allergy to latex may be the underlying cause.
Because the allergenic proteins associated with OAS are usually destroyed by cooking, most reactions are caused by eating raw foods. The main exceptions to this are celery and nuts, which may cause reactions even after being cooked.
Allergies to a specific pollen are usually associated with OAS reactions to other certain foods. For instance, an allergy to ragweed is associated with OAS reactions to banana, watermelon, cantaloupe, honeydew, zucchini, and cucumber. This does not mean that all sufferers of an allergy to ragweed will experience adverse effects from all or even any of these foods. Reactions may be associated with one type of food, with new reactions to other foods developing later. However, reaction to one or more foods in any given category does not necessarily mean a person is allergic to all foods in that group.
The patient typically already has a history of atopy and an atopic family history. Eczema, otolaryngeal symptoms of hay fever or asthma will often dominate leading to the food allergy being unsuspected. Often well-cooked, canned, pasteurized or frozen food offenders cause little to no reaction due to denaturation of the cross-reacting proteins, causing delay and confusion in diagnosis as the symptoms are elicited only to the raw or fully ripened fresh foods. Correct diagnosis of the allergen type/s involved is critical. OAS sufferers may be allergic to more than just pollen. Oral reactions to food are often mistakenly self-diagnosed by patients as caused by pesticides or other contaminants. Other reactions to food—such as lactose intolerance and intolerances which result from a patient being unable to metabolize naturally occurring chemicals (e.g., salicylates and proteins) in food—need to be distinguished from the systemic symptoms of OAS.
The cornerstone of diagnosis remains an accurate history of symptoms and an elimination diet followed by a food challenge. Skin prick testing and RAST testing are used as adjuncts to the clinical history—they cannot be used for diagnosis alone. Prick to prick testing with fresh foods is more reliable for some extremely labile allergens such as those found in apple than testing with commercial extracts which will commonly give a false negative. If the history is suggestive and the skin prick test negative, fresh foods should be used.
Many people have no idea that they have OAS. However, if swelling, tingling or pain develops while eating certain foods, then it is wise to see an allergy specialist. Before a diagnosis can be made, keep a food diary. This is important as the physician can then perform an allergy test. Before testing is started, a comprehensive history is obtained so that random testing is avoided. The diagnosis of OAS may involve skin prick tests, blood tests, patch tests or oral challenges. When OAS is suspected, the oral challenge test is ideal.
To confirm OAS, the suspected food is consumed in a normal way. The period of observation after ingestion and symptoms are recorded. If other co factors like combined foods are required, this is also replicated in the test. For example, if the individual always develops symptoms after eating followed by exercise, then this is replicated in the laboratory.
OAS must be managed in conjunction with the patient's other allergies, primarily the allergy to pollen. The symptom severity may wax and wane with the pollen levels. Published pollen counts and seasonal charts are useful but may be ineffective in cases of high wind or unusual weather, as pollen can travel hundreds of kilometers from other areas.
In addition, patients are advised to avoid the triggering foods, particularly nuts.
Peeling or cooking the foods has been shown to eliminate the effects of some allergens such as mal d 1 (apple), but not others such as celery or strawberry. In the case of foods such as hazelnut, which have more than one allergen, cooking may eliminate one allergen but not the other.
Antihistamines may also relieve the symptoms of the allergy by blocking the immune pathway. Persons with a history of severe anaphylactic reaction may carry an injectable emergency dose of epinephrine (such as an EpiPen). Oral steroids may also be helpful. Allergy immunotherapy has been reported to improve or cure OAS in some patients. Immunotherapy with extracts containing birch pollen may benefit OAS sufferers of apple or hazelnut related to birch pollen-allergens. Even so, the increase in the amount of apple/hazelnut tolerated was small (from 12.6 to 32.6 g apple), and as a result, a patient's management of OAS would be limited.