There are different classes of allergic reactions. Type I is most severe and can lead to anaphylactic shock and death; people can have this reaction to bee stings or shellfish. Type IV is a skin rash caused by latex or chemicals in latex gloves.
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. 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.
There are indoor allergens as well as outdoor allergens. Examples of indoor allergens include dust mites, mould, pet dander (from animals and birds) and cockroaches. Examples of outdoor allergens are pollens, grass and mould. We are listing only a few. Other substances such as cigarette smoke, perfumes and aerosol sprays are irritants which can worsen allergy and sinus symptoms.