ABSTRACT
The immune system is a destructive organization in the human body which constantly destroys invading microorganisms from the environment using mechanisms such as complement fixation, ADCC, cytotoxic T cells, and reactions involving many other cytotoxic factors. Therefore, it is not surprising that some of the immune reactions may actually damage host tissues due to an overresponse to certain antigens; this undesirable response of the immune system to the specific antigen, also known as allergen, is allergy. The subsequent immunological reaction after the exposure to the allergen is hypersensitivity; but, generally the terms allergy and hypersensitivity are used interchangeably in many occasions. Hypersensitivity can be considered as a result of the “double-edged sword” effect of the immune system and some of the hypersensitive responses possibly also play positive roles in the self-defense process. However, many reactions in hypersensitivity appear to have no apparent beneficial effect on the human body. It is very likely that during the evolution of the immune system, the infectious agents have changed drastically in the environment and, therefore, the functions of these immune reactions have become obsolete. For example, it is suggested that the anaphylactic response from allergic reaction is a critical mechanism for the defense against certain types of parasitic infections which may be uncommon today. In fact, it has been demonstrated that the production of IgE against parasites such as in schistosomiasis is an important event in humans who have acquired a high resistance to the parasitic infection.
