Allergies are abnormal reactions of the immune system that occur in response to otherwise harmless substances.
1 : altered bodily reactivity (as hypersensitivity) to an antigen in response to a first exposure
2 : exaggerated or pathological immunological reaction (as by sneezing, difficult breathing, itching, or skin rashes) to substances, situations, or physical states that are without comparable effect on the average individual
3 : medical practice concerned with allergies
4 : a feeling of antipathy or aversion
The term “allergy” was originally introduced by von Pirquet in 1906, meaning “changed reactivity”. At that time he imposed no restrictions as to the type of immunological response in the patient. It is only in recent years that “allergy” has become synonymous with Type I hypersensitivity (IgE-mediated allergy). Four different types of hypersensitivity were described by Coomb and Gell (Types I, II, III and IV), as a pedagogical way to increase the understanding of different immune reactions which could be provoked by many antigens. In practice these types do not necessarily occur in isolation from each other.
Allergic diseases generally begin in childhood, although they can arise at any age. Development of allergic disease is associated with an allergic constitution due to heredity and to environmental and health factors.
The allergic constitution is called atopic and reflects an increased production of allergen-specific IgE antibodies, which may lead to clinical symptoms such as rhinitis, asthma, eczema, colic pains or diarrhea. A state of hyperreactivity often accompanies an allergic reaction. If this hyperreactivity occurs in the respiratory tract, everyday stimuli like dust, tobacco smoke, cold air and perfumes may lead to allergy-like symptoms.
Description
Allergies are among the most common of medical disorders. It is estimated that 60 million Americans, or more than one in every five people, suffer from some form of allergy, with similar proportions throughout much of the rest of the world. Allergy is the single largest reason for school absence and is a major source of lost productivity in the workplace.
An allergy is a type of immune reaction. Normally, the immune system responds to foreign microorganisms or particles by producing specific proteins called anti-bodies. These antibodies are capable of binding to identifying molecules, or antigens, on the foreign particle. This reaction between antibody and antigen sets off a series of chemical reactions designed to protect the body from infection. Sometimes, this same series of reactions is triggered by harmless, everyday substances such as pollen, dust, and animal danders. When this occurs, an allergy develops against the offending sub-stance (an allergen.)
Mast cells, one of the major players in allergic reactions, capture and display a particular type of antibody, called immunoglobulin type E (IgE) that binds to allergens. Inside mast cells are small chemical-filled packets called granules. Granules contain a variety of potent chemicals, including histamine.
Immunologists separate allergic reactions into two main types: immediate hypersensitivity reactions, which are predominantly mast cell-mediated and occur within minutes of contact with allergen; and delayed hypersensitivity reactions, mediated by T cells (a type of white blood cells) and occurring hours to days after exposure.
Inhaled or ingested allergens usually cause immediate hypersensitivity reactions. Allergens bind to IgE anti-bodies on the surface of mast cells, which spill the contents of their granules out onto neighboring cells, including blood vessels and nerve cells. Histamine binds to the surfaces of these other cells through special proteins called histamine receptors. Interaction of histamine with receptors on blood vessels causes increased leakiness, leading to the fluid collection, swelling and increased redness. Histamine also stimulates pain receptors, making tissue more sensitive and irritable. Symptoms last from one to several hours following contact.
In the upper airways and eyes, immediate hypersensitivity reactions cause the runny nose and itchy, bloodshot eyes typical of allergic rhinitis. In the gastrointestinal tract, these reactions lead to swelling and irritation of the intestinal lining, which causes the cramping and diarrhea typical of food allergy. Allergens that enter the circulation may cause hives, angioedema, anaphylaxis, or atopic dermatitis.
Allergens on the skin usually cause delayed hypersensitivity reaction. Roving T cells contact the allergen, setting in motion a more prolonged immune response. This type of allergic response may develop over several days following contact with the allergen, and symptoms may persist for a week or more.