A bad reaction to a food doesn't necessarily mean you have a food allergy.
For a true food allergy to be present, two things must be happen:
The reaction must involve the immune system. Allergy tests determine if the immune system is involved. The reaction must create certain symptoms. Symptoms alone, however, do not prove allergy.A food allergy is a term that health experts reserve for any abnormal reaction by the body's disease-fighting immune system to an otherwise harmless food or component of food.Only a small fraction of the population has a genuine allergic reaction to food. In fact, in North America, only about 5 to 8 percent of children and 2 percent of adults have a true food allergy.When a reaction to a food occurs that does not involve the body's immune system, it is called food intolerance. This is not a food allergy.
 
Food intolerance stems from problems with digestion or metabolism. Usually the problem involves a defect or deficiency in an enzyme in the body, a chemical necessary for the breakdown or absorption of a particular food deficiency.

Medical experts agree that it's important to clear up the confusion surrounding food allergy and intolerance, because the potential consequences of such confusion can be serious.
While food intolerance can be unpleasant, it's rarely dangerous.
 
Individuals who have genuine food allergies, however, need to know which foods to avoid, how to recognize the symptoms of an allergic attack, and how to take steps to prevent or short-circuit a severe allergic reaction.These projections are called villi.
Relief of symptoms or reversion of an abnormal intestinal biopsy is the most convincing evidence that an individual has celiac disease or gluten sensitivity.
 
The immune system is a complex network of cells and molecules that help defend the body against foreign substances. When a properly functioning immune system detects a foreign substance, it responds to this threat by producing proteins called antibodies against the invaders. The antibodies will recognize and attack this foreign substance when they next encounter it. This "battle" is what causes the allergy symptoms.

 
In food allergy:

The immune system mistakenly sees a harmless substance in the food as harmful, and churns out antibodies-known as immunoglobulin E (IgE) - to attack it.

These antibodies will circulate in the blood, attached to special cells called mast cells, which are part of the immune system. This occurs in order to protect against future invasion by that substance. The next time a person eats that food, the substance to which he is allergic (the allergen) enters the body, and attaches to the IgE on mast cells.

The mast cells respond by releasing a host of powerful chemicals, including histamine, to 'protect' the body. This produces allergic symptoms.

Histamine contributes to inflammation and causes symptoms such as swelling on the skin and itching. It is responsible for
the hives, or welts, that appear on the skin when a doctor tests for allergy. These hives show the presence of IgE and are one of the best indications of allergy.

What Is Food Intolerance?
Very often, people mistake food intolerance for food allergy. Food intolerance is much more common than food allergy, and is the less serious of the two conditions.
Food intolerance is an exaggerated or abnormal physical reaction to a food or food additive (such as an artificial coloring or preservative) that does not involve an immune reaction. A chemical deficiency in the body is usually the cause of the problem.
It has nothing to do with food allergy because the reaction does not involve the body's immune system.

Causes of food intolerance
Several factors may cause a person to have an adverse reaction to food. Sometimes a person's body will react to chemicals found naturally in foods. For example, some people get a headache after eating certain cheeses and other foods that contain a compound called tyramine. In other cases, psychological factors play a role in food intolerance.

Carbohydrate intolerance. Some people cannot digest certain carbohydrates, compounds that furnish most of the energy needed in a healthy diet. Carbohydrates exist as simple sugars (sucrose, glucose, fructose, and lactose) or as glucose polymers (complex carbohydrates), such as glycogen and cellulose.

Celiac disease. This disorder is caused by an intolerance to gluten, the protein found in wheat and a few other grains. In celiac disease, the cells lining the small intestine are damaged and prevent the normal absorption of food constituents, particularly fats. Celiac disease involves an immune response. However, this response does not involve IgE, an antibody involved in the allergic response. Common symptoms are gas and bloating.

What Are The Symptoms Of Food Allergy?

In an allergy attack, the symptoms experienced depend on where in the body histamine is released. Allergic reactions to foods most often involve the skin, the stomach and intestines (digestive tract), and the mouth and airways (respiratory system). A life-threatening reaction may involve all parts of the body including the cardiovascular system so that the individual goes into shock (the blood pressure falls dangerously low). A severe reaction could start very suddenly and involve only a fall in blood pressure (shock). It is important to know that a life-threatening reaction may occur with no skin symptoms.

Symptoms may appear within minutes or as long as several hours after eating the allergy-provoking food.

Skin symptoms
Digestive symptoms
Respiratory symptoms
Life-threatening reactions (anaphylaxis)
Skin Symptoms
An allergic reaction may cause:
Hives (urticaria)
Itchy, scaly rash called eczema (or atopic dermatitis)
Redness or flushing (erythema)
Swelling (edema)
Digestive Symptoms

An allergy affecting the digestive system may cause:
Stomach pain
Nausea
Vomiting
Diarrhea

Respiratory Symptoms
Common respiratory symptoms from an allergic reaction include:
Sneezing
Coughing
Runny nose
Wheezing

Closing of the throat and breathing difficulties, as part of a severe allergic reaction known as anaphylaxis

Itchy, watery eyes are often included with respiratory symptoms.

Life-Threatening Reactions (Anaphylaxis)
The most severe allergic reaction is anaphylaxis, or anaphylactic shock, a severe reaction that involves most of the body. Anaphylaxis can affect several parts of the body at the same time, including the skin and the digestive and respiratory systems or it might just involve respiratory or cardiovascular symptoms.

In addition to producing the symptoms of food allergy, it may also lead to difficulty in breathing, falling blood pressure and unconsciousness.

Although very rare, anaphylaxis can be fatal. Each year, about 150 people in the United States die of food-related anaphylaxis.

Which Foods Cause Allergic Reactions?
Virtually any food can trigger an allergic response. However, studies have found that 80 to 90 percent of children with food allergies are allergic to one or more of these foods:

Eggs
Peanuts
Milk
Wheat
Soy
Other common triggers include:
Tree nuts (such as almonds, pecans, cashews, Brazil nuts and walnuts)
Fish and shellfish (for example, crab and shrimp)

Various other foods and certain food additives and spices may cause allergic reactions.


Glossary
Here are definitions of medical terms related to Food Allergies And Intolerance.

Additive: A substance, such as an artificial color or preservative, added to food.

Allergen: A substance that produces an allergic reaction.

Allergy: The body's exaggerated reaction to a foreign substance that is harmless to most people. For a reaction to be considered an allergic reaction, it must involve the immune system. The foreign substance may be eaten, inhaled or injected.

Anaphylaxis: A severe allergic reaction that involves much of the body-the respiratory system, skin, digestive tract and, sometimes, blood circulation. Symptoms include cough, wheezing, closing of the throat, runny nose, hives and swelling, nausea, vomiting, diarrhea, cramps and a drop in blood pressure that may cause loss of consciousness and even death. However it is important to know that a life-threatening or fatal reaction may occur without skin symptoms.

Antibody: A protein made by the body's immune system to combat foreign substance, most often a foreign protein.

Antihistamine: A medicine used to counteract histamine, a chemical released by the body in an allergic reaction, which contributes to the inflammation.

Atopic dermatitis: See eczema.

Biopsy: Removal of tissue for examination.

Cross contamination: When a small amount of food accidentally gets into another food.

Double-blind study: A trial in which neither the patient nor physician can distinguish between the real thing (for example a pill) and the inactive substance(the dummy pill).

Eczema (atopic dermatitis): A non-scarring skin rash that usually begins in childhood. Allergy may be involved in less than half of all children with eczema.

Enzyme: A protein that helps other chemical reactions occur, but isn't itself affected in the reaction.

Epinephrine (also known as adrenaline): A hormone, also used as a drug used to treat anaphylactic shock.

Histamine: A chemical in certain body cells, which contributes to inflammation and causes itching, swelling, sneezing and other allergic symptoms.

Hives: An allergic condition characterized by red, slightly swollen eruptions or itchy lumps on the skin.
hypersensitivity: Another word for allergy.

IgE (Immunoglobulin E): One type of antibody that defends the body against outside invaders. When certain proteins interact with specidic IgE antibodies the resulting reaction is an allergic reaction.

Immune system: The body's defense system, which fights disease and infections.

Immunoglobulin: A group of proteins known as antibodies that are involved in defending the body against outside invaders.

Inflammation: A protective reaction to an injury that may cause severe discomfort.

Mast cells: Cells present close to body surfaces (such as the skin and intestines), which produce and release chemicals-for instance, histamine-which then produce allergic symptoms.

Metabolism: The building and breaking that occurs continually in the body.

Placebo: A substance used in testing that has no known physical effect on the body. It may have a powerful psychological effect, however.

Toxin: A poison.

Vaccine: An injection given to induce immunity and protect against a certain germ.
 

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