An allergy is an abnormal sensitivity to an allergen that is inhaled, eaten or touched that most other people can tolerate with no trouble. Allergies are over-reactions by someone to substances their immune system considers harmful even if they are not. A normal reaction should be the development of immunity. Our immune system is there to defend us from any aggression or invasion of foreign substances.

Its means of defense are sophisticated and complex. They consist of producing antibodies, which are our body's "defenders". Their job is to destroy intruders, or so-called antigens. The antibodies involved in allergies are known as IgE (Immunoglobulin E). Everyone makes some IgE - only the people with genetic predisposition towards allergies make large quantities. Whereas, IgG antibodies rid the body of infections, like a strep throat & viruses, IgE antibodies have misdirected attacks against pollens, dustmite, dander, and moulds. These immunological (involving the immune system) reactions should be theoretically beneficial. But, in some cases, our immune system goes overboard and "overdoes it", or loses control, reacting to harmless substances that pose no danger or threat to the individual.

This over-reaction and loss of control by the immune system is seen in people with allergies, where the immune system launches attack against a perfectly harmless substance, such as grass pollen, cat dander or peanut or penicillin. The harmful end results of these attacks are called immediate hypersensitivity or allergy. The antigen responsible for an allergic reaction is called an allergen. The other main components to allergies are mast cells, which contain chemicals like histamine. The IgE Antibodies sit on the surface of the mast cells. A mast cell has about 1,000 histamine containing packets (granules) in its cytoplasm and on its surface are between 100,000 and 1 million receptors for IgE. When the IgE encounters allergen, it triggers the mast cell to release granules from its cytoplasm. Those granules contain histamine and other chemicals. These mediators that are released then interact with the lining of the site involved (skin, nose, eyes, lung etc), causing the allergic symptoms.

The word allergy comes from the Greek allos, meaning other. It was first used in 1906 to refer to "altered reaction" in the body's immune system. Since then the term allergy has been used to describe a host of conditions most of which have nothing to do with the immune system. For someone to have an allergic reaction, they have to be sensitized to the allergen. Being sensitized means that the immune system has been in contact with an antigen, that it has committed it to memory and has produced specific antibodies against it. At a later contact, it will recognize the antigen and immediately react against it. Our immune system has a good memory. From the first time the immune system comes in contact with a foreign substance, it learns how to recognize it and memorizes it. That is why someone suffering from hay fever will react every time he comes in contact with that specific type of pollen that has been memorized by his immune system.

True allergic reactions occur within minutes to an hour of contact with the allergen. This differentiates them from pseudo-allergic reactions, which do not involve the immune system and can occur up to 48 hours after contact with the offending substance. Some people use the term intolerance to refer to some of the non-immunologic adverse reactions to foods.

Allergic reactions are caused by substances in the environment known as allergens. Almost anything can be an allergen for someone. Allergens contain protein, which is often regarded as a constituent of the food we we eat.

In fact it is an organic compound, containing hydrogen, oxygen and nitrogen, which form an important part of living organisms.

The most common allergens are:

pollen from trees and grasses, house dust mite, moulds, pets such as cats and dogs, insects like wasps and bees, industrial and household chemicals, medicines, and foods such as milk and eggs.

Less common allergens include nuts, fruit and latex.

There are some non-protein allergens which include drugs such as penicillin. For these to cause an allergic response they need to be bound to a protein once they are in the body.

An allergic person's immune system believes allergens to be damaging and so produces a special type of antibody (IgE) to attack the invading material. This leads other blood cells to release further chemicals (including histamine) which together cause the symptoms of an allergic reaction.

The most common symptoms are:

sneezing , runny nose, itchy eyes and ears, severe wheezing, coughing shortness of breath, sinus problems, a sore palate and nettle-like rash.

It should be understood that all the symptoms mentioned can be caused by factors other than allergy. Indeed some of the conditions are diseases in themselves.

When asthma, eczema, headaches, lethargy, loss of concentration and sensitivity to everyday foods such as cheese, fish and fruit are taken into account the full scale of allergy be appreciated.

There are two types of environmental allergens:

Seasonal: These are mainly outdoor allergens and include pollens from grasses, trees and weeds, and also some moulds.

Perennial: These are usually indoor allergens, and cause symptoms all year round. The commonest is the house dust mite. These reside in nests such as carpets, bedding and upholstered furniture. House dust mites proliferate in humid homes. They are the commonest triggers for perennial rhinitis and allergic asthma.

Other perennial allergens include domestic pets, especially cats. The actual allergen is a protein component of dander, the shed scales of the skin. The same allergens are also found in the animal"s saliva and urine. Hair on fur can also collect other allergens, such as pollens, moulds and dust mites.

Only about six foods account for more than 90% of all confirmed food allergies. These are milk, eggs, peanut & other nuts, wheat, soy and fish.

Aspirin & other anti-inflammatory drugs and antibiotics like penicillin account for most of the allergic reactions due to drugs.

Each year about 40 people die in the USA from Anaphylaxis to insect (bee, wasp & fire ants venom) sting.

Over the last 15 years immediate hypersensitivity (IgE-mediated allergy) to latex has been increasingly recognized as a rapidly emerging public health problem. Latex is widely used in the manufacture of medical devices (gloves, catheters, anaesthetic masks, and dental dams) as well as a variety of everyday articles. Reactions from latex range from annoying skin rashes to life-threatening anaphylaxis.

To confirm which allergen is responsible, skin prick tests may be recommended using extracts from allergens such as house dust mites, pollens, or moulds commonly found in the local area. A diluted extract of each kind of allergen is pricked into the skin of the forearm or on the back.

With a positive reaction, a small, raised, reddened area with surrounding flush (called a wheal and flare) will appear at the test site. The results are measured after 15 minutes. The size of the wheal can provide the physician with an important diagnostic clue, but a positive reaction does not prove that a particular allergen is the cause of the patient"s symptoms. It proves that IgE antibody to that specific allergen is present in the skin.

Diagnostic tests can be done using a blood sample from the patient to detect levels of IgE antibody to a particular allergen. One such blood test is called the RAST (radioallergosorbent test), which can be performed when the entire body is covered with eczema or if the patient is on antihistamines, which will interfere with skin prick test results.

If diagnosis of food sensitivity remain in doubt after the patient has used diet diaries, eliminated certain foods, and undergone allergy tests, the allergist may recommend "blinded" food and/or food additive "challenges" in the office or hospital under close observation. Usually, the suspected food or a neutral food, called a placebo, is fed in opaque colorless capsules, or as a hypoallergenic slush or pudding so neither the patient nor the doctor knows whether the suspected food or placebo is being eaten. This is called "double-blind" challenge and when properly performed is helpful in establishing the cause-and-effect relationship between a food and an allergy symptom. This is considered by some to be the gold standard for diagnosing any adverse reaction to foods.