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Allergy is a hypersensitivity disorder characterized by an exaggerated immunologic response to an otherwise innocuous agent, called an allergen. Allergies can be caused by both host (race, gender, hereditary) and environmental factors (pollution, diet, infection).
The increase in allergic diseases industrialized countries over the past decades has been attributed to a decline in infections during childhood, also known as the hygiene hypothesis.
The immunological allergic response is often thought of in terms of the Gell-Coombs classification which states that there are four main types of hypersensitivity.
The most common allergic response is Type I. IgE antibodies are produced in response to an allergen in the sensitization phase and antigens are presented to T-helper (Th2) cells. Th2 cell produce cytokines such as IL-3, IL-4 and IL-5 which promote development and survival of other immune cells such as B cells and eosinophils. IgE antibodies then attach to mast cells and basophils and upon a secondary exposure to allergen, antigen-antibody binding will occur at these sites. Crosslinking of IgE on mast cells leads to their degranulation and the release of mediators responsible for the allergic reaction. These mediators increase mucus secretion and promote vascular permeability and smooth muscle contraction. Eosinophils, neutrophils and monocytes are recruited in the late phase and release additional mediators to sustain the allergic response.