Link to original: http://sphynxcatvp.nocturna.org/health/health-foodallergies.html
How allergic reactions work
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 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. Food allergies are usually inherited - if allergies of any kind are common in a family, then developing food allergies will be more likely, especially if both parents have allergies of some kind.
Before an allergic reaction can occur, the person has to be exposed to the foods s/he is allergic to. As this food is digested, it triggers the allergic response, generating these antibodies in large amounts which then attach to these mast cells. The next time the person eats that food, it interacts with those antibodies and triggers the mast cells to release chemicals such as histamine. The location of the body these cells are in will determine the location of the allergic response. For example, if these cells are in the intestines, the person may have abdominal pain or diarrhea. The chemicals released by mast cells in the skin can cause hives.
The digestion process affects the timing and the location of a reaction. The initial reaction may be a tingling or itching sensation as they start to eat the food, then reactions may occur in the stomach and abdominal region, and as the allergens travel through the bloodstream, possibly a drop in blood pressure, hives or eczema may occur as well, or even trigger an asthma attack when the allergens reach the lungs. All or part of these types of reactions can occur between near-instantaneous to an hour or so after ingesting the offending food.
Although an individual could be allergic to any food, such as fruits, vegetables, and meats, they are not as common as the following eight foods which account for 90% of all food-allergic reactions: Milk, Egg, Peanuts (one of the chief foods that cause severe anaphylaxis), Tree nut (walnut, cashew, etc.), Fish, Shellfish (shrimp, crayfish, lobster, crab and so on), Soy and Wheat (gluten).
Other allergens include medications, insect stings (such as bees), and latex (particularly common among healthcare workers.)
Food allergies vary in severity, from life threatening to just a nuisance rash. Reactions are sometimes dose-dependent: trace or small amounts may be fine, but larger amounts may trigger a reaction, however severe. If you are allergic to one specific food in a food family other foods in the same food family (i.e., dairy products) may cause a similar reaction, even if you're not already aware of it. The foods that adults or children react to are usually those foods they eat more often. In Japan, for example, rice allergy is more frequent. In Scandinavia, codfish allergy is more common.
If someone has a life-threatening reaction to a certain food, doctors will usually advise them to avoid similar foods. For example, a history of allergy to shrimp will usually indicate cross-reactivity to other shellfish such as lobster or crab as well.
During pollen seasons, people allergic to particular pollens or ragweed may find they have a reaction when eating melons or fruits. This is likely due to pollen dust resting on the skin of the fruit, or when the fruit is sliced (such as cantaloupe), some of the pollen dust then gets dragged into the rest of the fruit on the surface of the knife.
A sudden, severe, potentially fatal, systemic allergic reaction that can involve various areas of the body (such as the skin, respiratory tract, gastrointestinal tract, and cardiovascular system). Symptoms occur within minutes to two hours after contact with the allergy-causing substance, but in rare instances may occur up to four hours later. Anaphylactic reactions can be mild to life-threatening.
Common causes of anaphylaxis include: Food (such as peanuts), medications, insect stings (bee stings being a common severe allergen), and latex (particularly common among healthcare workers). Anyone with a history of anaphylactic reactions is at risk for another severe reaction.
While "allergy" and "intolerance" are often used interchangeably, they are really two different things. A food intolerance is an adverse food-induced reaction that does NOT involve the immune system - lactose intolerance is one example, or MSG sensitivity, or reactions to things like Nutrasweet. When a lactose intolerant person eats or drinks something that is milk based, symptoms such as gas, bloating, or abdominal pain may occur because the person's body lacks an enzyme that is needed to digest milk products. This usually does not always have the potential to be fatal, merely inconvenient, painful or embarrassing.
Sometimes food poisoning symptoms will mimic the symptoms of a food allergy.
Another intolerance example is an adverse reaction to certain components that are included in food to enhance taste, color, or preserve it. Such substances can include artificial colors and flavors (One reaction to artificial colors/flavors in foods, particularly in children, is "hyperactivity" or attention deficit disorder. It's not, however, the only ADD cause.) MSG and sulfites are other items that can generate an intolerant reaction.
Gluten sensitivity, or Celiac Syndrome, is another common adverse reaction - this is cause by an abnormal immune response to gluten, which is a component in wheat and other grains. Because gluten and gluten byproducts make their way into a LOT of foods, someone with this problem has to spend a lot of time reading labels to be sure there's not wheat or gluten products.
Discovering food allergies
The first step is to find out what you're allergic to. Keep a food diary for at least 1-2 weeks. Log any and all negative reactions you get, regardless of when they happen. Log any and all foods you eat, drink or snack on, even if it's just "a nibble of so-and-so's sandwich". Log anything you eat or drink in-between meals as well. Log all medication you may be taking (sometimes a medication taken on an empty stomach will give you abdominal pains, but be OK once you've had food) and when you take it. This information can tell you (or your doctor) what you may be allergic to.
Questions to keep in mind when keeping a food diary (and your doctor will most likely ask similar questions, if not more) are:
What was the timing of the reaction? Did the reaction come on quickly, usually within an hour after eating the food?
Is the reaction always associated with a certain food?
Did anyone else get sick? For example, if the person has eaten contaminated fish, everyone who ate the fish should be sick. In an allergic reaction, however, only the person actually allergic to the fish becomes ill.
How much did the patient eat before experiencing a reaction? The severity of the patients reaction is sometimes related to the amount of food the patient ate.
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.
The next step is a true elimination diet. Pick a food or food group (i.e., milk or dairy products) and eliminate it for two weeks to a month. Observe your reactions, whether they change or stay the same. After this period of time, put that food/food group back in your diet and eliminate another food/food group (i.e., shellfish) for another two weeks to a month. Again, observe your reactions, whether they change or stay the same. Repeat this until you've worked through all the foods you usually eat, or you find out what causes a reaction, whichever comes first.
Once you've gone through the elimination diet, you should know what foods you're allergic to, or if you're allergic to any foods. (Sometimes what can seem like a food allergy can be a case of food poisoning due to bad food preparation/storage in a restaurant.)
Treating food allergies
Food allergies are easily (if annoyingly) treated by dietary avoidance - once the food allergens are identified, the food must be removed from the person's diet. This requires reading of lengthy ingredient lists for each item they are considering eating. Foods such as peanuts, eggs and milk often show up in foods that would not normally be associated with them, or there are trace amounts stemming from "shared equipment" (the equipment is used for multiple types of food) used in the preparing factory.
I will admit it IS a pain in the ass to read every single label to find the allergens you react to - however, the end result of a more comfortable life IS worth the trouble.
People that are highly allergic to an item (such as peanuts) cannot tolerate even the smallest amounts without having a reaction. People who's allergies are mild to moderate can tolerate small amounts with little to no discomfort.