Allergy

Immunology

An allergic reaction is classified as a type I hypersensitivity reaction, characterized by secretion of immunoglobulin E (IgE) antibodies. In an allergic reaction, a person must be previously sensitized to the particular allergen, so that the immune system is able to identify and memorize a specific molecular “footprint” for each allergen. On encountering the allergen for the first time, IgE is produced. These IgE antibodies attach themselves to mast cells, which line the organs of the body, or basophils, which are present in blood. Mast cells and basophils contain granules full of inflammatory molecules, including histamine, leukotrienes, and prostaglandins.

Allergies can affect many organ systems, including airways, eyes, nose, and skin, and manifest in many ways, including hay fever.

[Page 61]Other immune cells also play an important role in allergy and the allergic reaction. Activated B cells (or plasma cells) produce the IgE antibodies selectively in allergic individuals. B cells are also able to make other antibodies (including IgG) against allergens that do not lead to mast cell degranulation but are protective against allergic inflammation. Allergen-specific IgG antibody, but not IgE, has been found in nonallergic individuals, demonstrating that their immune systems can also recognize and respond to the allergen, but without inducing an inflammatory response.

When a previously sensitized person comes in contact with the same allergen for the second time, the allergen crosslinks two or more molecules of IgE on the same mast cell or basophil, leading to degranulation. The release of granules causes the effects of inflammation, either systemically or locally at the site of exposure (lungs, skin, etc.). These can include redness, swelling, itchiness, increased vascular permeability, mucus secretion, and anaphylaxis.

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