The mechanism behind an allergic reaction involves two features of the human immune response. One is the production of immunoglobulin E (IgE), a type of protein called an antibody that circulates through the blood. The other feature of the immune response is the mast cell, a specific cell that occurs in all body tissues but is especially common in areas of the body that are typical sites of allergic reactions, including the nose and throat, lungs, skin, and gastrointestinal tract.
The ability of a given individual to form IgE against something as benign as food is an inherited predisposition. Generally, such people come from families in which allergies are common, not necessarily food allergies but perhaps hay fever, asthma, or hives. Someone with two allergic parents is more likely to develop food allergies than someone with one allergic parent.
Before an allergic reaction can occur, a person who is predisposed to form IgE to foods first has to be exposed to the food. As this food is digested, it triggers certain cells to produce specific IgE in large amounts. The IgE is then released and attaches to the surface of mast cells. The next time the person eats that food, it interacts with specific IgE on the surface of the mast cells and triggers the cells to release chemicals such as histamine. Depending upon the tissue in which they are released, these chemicals will cause a person to have various symptoms of food allergy.
If the mast cells release chemicals in the ears, nose, and throat, a person may feel an itching in the mouth, and may have trouble breathing or swallowing. If the affected mast cells are in the gastrointestinal tract, the person may have abdominal pain or diarrhea. The chemicals released by skin mast cells, in contrast, can prompt hives.
Food allergens (the food fragments responsible for an allergic reaction) are proteins within the food that usually are not broken down by the heat of cooking or by stomach acids or enzymes that digest food. As a result, they survive to cross the gastrointestinal lining, enter the bloodstream, and go to target organs, causing allergic reactions throughout the body.
The complex process of digestion affects the timing and the location of a reaction. If people are allergic to a particular food, for example, they may first experience itching in the mouth as they start to eat the food. After the food is digested in the stomach, abdominal symptoms such as vomiting, diarrhea or pain may start. When the food allergens enter and travel through the bloodstream, they can cause a drop in blood pressure. As the allergens reach the skin, finally, they can induce hives or eczema. All of this takes place within a few minutes to an hour.
Common Food Allergies
In adults, the most common foods to cause allergic reactions include: shellfish, such as shrimp, crayfish, lobster, and crab; peanuts, which is one of the chief foods to cause severe anaphylactic reactions; tree nuts, such as walnuts; fish; and egg.
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In children, the pattern is somewhat different. The most common food allergens that cause problems in children are egg, milk, and peanuts.
The foods that adults or children react to are those foods they eat often. In Japan, for example, rice allergy is more frequent. In Scandinavia, codfish allergy is common.
Cross Reactivity
If someone has a life-threatening reaction to a certain food, the doctor will counsel the patient to avoid similar foods that might trigger this reaction. For example, if someone has a history of allergy to shrimp, testing will usually show that the person is not only allergic to shrimp, but also to crab, lobster, and crayfish, as well. This is called cross reactivity.
Another interesting example of cross reactivity occurs in people who are highly sensitive to ragweed. During ragweed pollination season, these people sometimes find that when they try to eat melons, in particular cantaloupe, they have itching in their mouth and they simply cannot eat the melon. Similarly, people who have severe birch pollen allergy also may react to the peel of apples.
Adults usually do not lose their allergies, but children can sometimes outgrow them. Children are more likely to outgrow allergies to milk or soy than allergies to peanuts, fish or shrimp.
Differential Diagnoses
A differential diagnosis means distinguishing food allergy from food intolerance or other illnesses. If a patient goes to the doctor's office and says, "I think I have a food allergy," the doctor has to consider the list of other possibilities that may lead to symptoms that could be confused with food allergy.
One possibility is the contamination of foods with microorganisms, such as bacteria, and their products, such as toxins. Contaminated meat sometimes mimics a food reaction when it is really a type of food poisoning.
There are also natural substances, such as histamine, that can occur in foods and stimulate a reaction similar to an allergic reaction. For example, histamine can reach high levels in cheese, some wines, and in certain kinds of fish, particularly tuna and mackerel. In fish, histamine is believed to stem from bacterial contamination, particularly in fish that hasn't been refrigerated properly. If someone eats one of these foods with a high level of histamine, that person may have a reaction that strongly resembles an allergic reaction to food. This reaction is called histamine toxicity.
Another cause of food intolerance that is often confused with a food allergy is lactase deficiency. This most common food intolerance affects at least one out of ten people. Lactase is an enzyme that is in the lining of the gut. This enzyme degrades lactose, which is in milk. If a person does not have enough lactase, the body cannot digest the lactose in most milk products. Instead, the lactose is used by bacteria, gas is formed, and the person experiences bloating, abdominal pain, and sometimes diarrhea.
Another type of food intolerance is an adverse reaction to certain products that are added to food to enhance taste, provide color, or protect against growth of microorganisms. Compounds that are most frequently tied to adverse reactions that can be confused with food allergy are yellow dye number 5, monosodium glutamate, and sulfites. Yellow dye number 5 can cause hives, although rarely. Monosodium glutamate (MSG)) is a flavor enhancer, and, when consumed in large amounts, can cause flushing, sensations of warmth, headache, facial pressure, chest pain or feelings of detachment in some people. These transient reactions occur rapidly after eating large amounts of food to which MSG has been added.
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Sulfites can occur naturally in foods or are added to enhance crispness or prevent mold growth. Sulfites in high concentrations sometimes pose problems for people with server asthma. Sulfites can give off a gas called sulfur dioxide, which the asthmatic inhales while eating the sulfited food. This irritates the lungs and can send an asthmatic into severe bronchospasm, a constriction of the lungs. Such reactions led the U.S. Food and Drug Administration (FDA) to ban sulfites as spray-on preservatives in fresh fruits and vegetables. But they are still used in some foods and are made naturally during the fermentation of wine, for example.
There are a number of other diseases that share symptoms with food allergies including ulcers and cancers of the gastrointestinal tract. These disorders can be associated with vomiting, diarrhea or cramping abdominal pain exacerbated by eating.
Some people may have a food intolerance that has a psychological trigger. In selected cases, a careful psychiatric evaluation may identify an unpleasant event in that person's life, often during childhood, tied to eating a particular food. The eating of that food years later, even as an adult, is associated with a rush of unpleasant sensations that can resemble an allergic reaction to food.
Diagnosis
To diagnose food allergy a doctor must first determine if the patient is having an adverse reaction to specific foods. This assessment is made with the help of a detailed patient history, the patient's diet diary, or an elimination diet.
The first of these techniques is the most valuable. The physician sits down with the person suspected of having a food allergy and takes a history to determine if the facts are consistent with a food allergy. The doctor asks such questions as:
What was the timing of the reaction?
Did the reaction come on quickly, usually within an hour after eating the food?
Was allergy treatment successful? (Antihistamines should relieve hives, for example, if they stem from a food allergy.)
Is the reaction always associated with a certain food?
Did anyone else get sick? For example, if the person has eaten fish contaminated with histamine, everyone who ate the fish should be sick.
However, in an allergic reaction, only the person allergic to the fish becomes ill.
How much did the patient eat before experiencing a reaction? The doctor will want to know how much you ate each time and try to relate it to the severity of the reaction.
How was the food prepared? Some people will have a violent allergic reaction only to raw or undercooked fish. Complete cooking of the fish destroys those allergens in the fish to which they react. If the fish is cooked thoroughly, they can eat it with no allergic reaction. Were other foods ingested at the same time of the allergic reaction? Some foods may delay digestion and thus delay the onset of the allergic reaction.
Sometimes a diagnosis cannot be made solely on the basis of history. The doctor may also ask the patient to go back and keep a record of the contents of each meal and whether he or she had a reaction. This gives more detail from which the doctor and the patient can determine if there is consistency in the reactions.
The next step some doctors use is an elimination diet. Under the doctor's direction, the patient does not eat a food suspected of causing the allergy, like eggs, and substitutes another food in this case another source of protein. If the patient removes the food and the symptoms go away, a diagnosis can almost be made. If the patient then eats the food (under the doctor's direction) and the symptoms come back, then the diagnosis is confirmed. This technique cannot be used, however, if the reactions are severe (in which case the patient should not resume eating the food) or infrequent.
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If the patient's history, diet diary or elimination diet suggest a specific food allergy is likely, the doctor will then use tests that can more objectively measure an allergic response to food. One of these is a scratch skin test, during which a dilute extract of the food is placed on the skin of the forearm or back. This portion of the skin is then scratched with a needle and observed for swelling or redness that would indicate a local allergic reaction. If the scratch test is positive, the patient has IgE on the skin's mast cells that is specific to the food being tested.
Skin tests are rapid, simple and relatively safe. But a patient can have a positive skin test to a food allergen without experiencing allergic reactions to that food. A diagnosis of food allergy is made only when a patient has a positive skin test to a specific allergen and the history of their reactions also suggests an allergy to the same food.
In some extremely allergic patients who have severe anaphylactic reactions, skin testing can't be used because it could evoke a dangerous reaction. Skin testing also cannot be done on patients with extensive eczema.
For these patients a doctor may use one of two blood tests called RAST and ELISA.These tests measure the presence of food-specific IgE in the blood of patients. These tests may cost more than skin tests and results are not immediately available. As with skin testing, positive tests do not necessarily make the diagnosis.
The final method used to objectively diagnose food allergy is double-blind food challenge. This testing has come into vogue over the last few years as the "gold standard" of allergy testing. For this food challenge, various foods, some of which are suspected of inducing an allergic reaction, are each placed in individual opaque capsules. The patient is asked to swallow a capsule and is then watched to see if a reaction occurs. This process is repeated until all the capsules have been swallowed. In a true double-blind test, the doctor is also "blinded," the capsules having been made up by some other medical person, so that neither the patient nor the doctor knows which capsule contains the allergen.
The one strong advantage of such a challenge is that, if the patient has a reaction only to suspected foods and not to other foods tested, it confirms the diagnosis. However, someone with a history of severe reactions cannot be tested this way. In addition, this testing is expensive because it takes a lot of time to perform. Multiple food allergies are also difficult to evaluate with this procedure.
Consequently, double-blind food challenges are not done often. This type of testing is most commonly used when the doctor believes that the reaction a person is describing is not due to a specific food and wishes to obtain evidence to support this judgment so that additional efforts may be directed at finding the real cause of the reaction.
