Allergy is a disease of the immune system characterized by overreaction brought by particular antibodies (reagin or IgE ) against normally harmless substances such as pollen.
The concept of "allergy" was introduced in 1906 by pediatricians Viennese Clemens von Pirquet and Bela Schick who were the first to observe how the immune system could also play a harmful role with altered responses after administration of heterologous serum or smallpox vaccine for therapeutic purposes. Pirquet and Schick called this phenomenon "allergy", words from greek ἄλλος, which means "other", and ἔργον meaning "work" . At first all forms of ' hypersensitivity were classified as allergies, because they thought they were all caused by an altered immune system activation. It became clear later that they were involved many different mechanisms of disease, having in common an abnormal immune system activation. In year 1963, PhilipGell and RobinCoombs proposed a new classification scheme which provided for 4 types of the hypersensitivity reactions, identified as hypersensitivity grade I to IV . With this new-fangled arrangement, the term "allergy" was controlled to merely one type I hypersensitivity
Key features are the specificity and speed.
Specificity: it is due to reaction of a type of antibody to the disease occurs only in the presence of the substance to which the antibody is directed. The same kind substances (eg pollen) but different molecular structure (eg pollen of 2 different families) generate different allergies. To summarize a patient with an allergy to some pollen has no symptoms when in contact with pollen from other families and structurally different.
Speed: The allergic reaction is immediate by definition. From the moment of contact with the allergenic substance at the time elapsing from the onset of symptoms 5 to 30 minutes (median 15 minutes).