Type I Hypersensitivity and it’s Mechanism

Type I hypersensitivity is an immediate anaphylactic reaction associated with allergy. These reactions are mediate by Ig-E antibodies and includes many of the most common allergies to respiratory allergens, such as pollen, dust mites etc. The mast cells and basophil, also participate in this reaction. Type I Hypersensitivity is also known as immediate or allergic hypersensitivity.

Mentioned disorders: Atopy, Asthma, Anaphylaxis etc.

Mediators: This hypersensitivity reaction is mainly mediated by Ig-E antibodies, Mast cells and Basophil cells.

Description or Mechanism of Reaction: It is an immediate allergic reaction as the immune response is very fast and occur in minutes, rather than multiple hours or days.

Exposure to an allergen activates the helper T-cells (TH2) which further activate the B-cells to develop into Ig-E secreting plasma B-cells. Ig-E antibodies are secreted from those plasma B-cells containing specific knowledge about these allergens. Ig-E antibodies alone are not destructive. They cause these types of immune response after binding to the Ig-E specific Fc-receptor, expressed by a variety of innate immune cells, such as mast cells, basophils, eosinophils.

Second exposure to the allergen leads to the cross-linking of bound Ig-E on mast cells and basophil cells. Binding of Ig-E antibodies to the Fc-receptor activates these granulocytes (mast cells, basophils) and induce a signalling cascade that causes cells to release the contents of the intracellular granules into the blood. This process is called degranulation.

From these intracellular granules some biomolecule is secreted, including histamine, heparin, and some protease enzymes. But the vasoactive biomolecule (amine), also secreted from those granules of mast cells and basophils causes smooth muscle contraction and increase the vascular permeability and also induces the vasodilation process and plays an vital role in this type of hypersensitivity reaction.   

Type I Hypersensitivity reaction is also an immune mediated disorder of our body’s normal immune system. The testing of this disorder can be done via measuring the level of specific Ig-E antibodies into the blood.

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