ALLERGY (Mechanism) | Detailed & Conceptional Explanation | Clear Diagrams

An allergy [Greek allos, other and ergon, work] is one kind of
type I hypersensitivity reaction. Allergic reactions occur when an
individual who has produced IgE antibody in response to the initial exposure to an antigen (allergen) subsequently encounters the
same allergen. Upon initial exposure to a soluble allergen, B cells
are stimulated to differentiate into plasma cells and produce specific IgE antibodies with the help of TH cells. This
IgE is sometimes called a reagin, and the individual has a hereditary predisposition for its production. Once synthesized, IgE binds
to the Fc receptors of mast cells (basophils and eosinophils can
also be bound) and sensitizes these cells, making the individual
sensitized to the allergen. When a subsequent exposure to the allergen occurs, the allergen attaches to the surface-bound IgE on the
sensitized mast cells, causing mast cell degranulation.
Degranulation releases physiological mediators such as histamine, leukotrienes, heparin, prostaglandins, PAF (platelet-activating factor), ECF-A (eosinophil chemotactic factor of anaphylaxis), and proteolytic enzymes. These mediators trigger
smooth muscle contractions, vasodilation, increased vascular
permeability, and mucus secretion. The inclusive
term for these responses is anaphylaxis [Greek ana, up, back
again, and phylaxis, protection]. Anaphylaxis can be divided into
systemic and localized reactions.
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