Type I hypersensitivity reaction occurs through two phases; the sensitization and effector phases, both occurring with an interval of 2–3 weeks (Fig. 1).

Fig1. Mechanism of type I hypersensitivity reaction.

 Sensitization Phase

This occurs when an individual is exposed for the first time to the sensitizing or priming dose of an allergen.

- Sensitization is most effective when the allergen is introduced parenterally, but may occur by any route, including ingestion or inhalation

- In susceptible individuals, very minute doses can be sufficient to sensitize the host - The allergen is processed by the antigen presenting cells and the antigenic peptides are presented to the CD4 helper T cells

- Activated TH cells are differentiated into TH2 cells which in turn secrete interleukin 4 (IL-4)

- IL-4 induces the B cells to differentiate into IgE producing plasma cells and memory cells. Many molecules of IgE with specificities against various epitopes of the allergen may be produced

- Secreted IgE migrate to the target sites, and coat on the surface of mast cells and basophils. Fc region (the CH 3 and CH 4 domains) of IgE binds to high affinity Fc receptors (e.g. FcεR1) present on mast cell surface

- Such sensitized mast cells (coated with IgE) will be waiting for interaction with the subsequent antigenic challenge.

Effector Phase

 When the same allergen is introduced subsequently (shocking dose), it directly encounters with the Fab region of IgE coated on mast cells.

- IgE cross linkage initiates degranulation: Allergen bound to IgE triggers the mast cells (and basophils) activation and degranulation. Granules in turn release a number of pharmacologically active chemical mediators that lead to the various manifestations of type-1 reaction

- The memory B cells further differentiate into plasma cells that produce IgE

- Degranulation in two phases: Mast cells and basophils undergo degranulation in two phases

1. Primary mediators: The preformed chemical mediators which are already synthesized by mast cells, are immediately released, e.g. histamine and serotonin (Table 1)

2. Secondary mediators: The mast cells synthesize them following stimulation by allergen and release, e.g. prostaglandins and leukotrienes (Table 1).

- Pharmacological actions: The chemical mediators perform several pharmacological actions, such as bronchial and other smooth muscle contraction, increased vascular permeability and vasodilation (Table 1)

- Symptoms: These actions in combinations, produce symptoms such as breathlessness, hypotension and shock leading to death at times.

Table1. Mediators of type i hypersensitivity.

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مواضيع ذات صلة

Detection of Type I Hypersensitivity

Manifestations of Type I Reaction

Self-Recognition (Tolerance)

Hypersensitivity Reactions

Humoral/Antibody-Mediated Immune Response

Cell-Mediated Immune Response

Helper T Cells (Activation AND Differentiation)

Type III (Immune Complex) Reactions

Influenza virus immunology

Type II Antibody-Dependent, Complement-Mediated Cytotoxic Reactions

Immunologic Manifestations of Rubella infection

Antigen presentation