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 not 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.