Chapter 14, Problem 1CT

Summary Introduction

To trace:

The path of pollen grain through sensitization and provocation in type I allergies discussing the role of mast cells, basophils, IgE, and allergic mediators.

Concept introduction:

Type I hypersensitivity or immediate hypersensitivity is triggered by the allergens and the person become “sensitized” by generating large amount of IgE in response to particular allergen. The initial introduction to allergen results in attachment of IgE to cell membrane of basophils and mast cells. The subsequent exposure rapidly release mediators like leukotrienes, and histamine and occurs within seconds. Therefore this reaction is termed as “immediate reaction”.

Explanation of Solution

Pictorial representation: The path of pollen grain through sensitization and provocation in type I allergy is represented as below:

Fig. 1: The path of pollen grain through sensitization and provocation in type I allergy.

All the type I hypersensitivity reactions have a common mechanisms which is immediate onset of allergic reactions against specific antigens. Given below are the roles of mast cells, basophils, IgE, and allergic mediators:

1. IgE: Type I hypersensitivity is triggered by the allergens such as pollen and the person become “sensitized” by generating large amount of IgE in response to particular allergen. The initial introduction to allergen results in attachment of IgE to cell membrane of basophils and mast cells.

2. Mast cells: Mast cells are located in the mucosal and epithelial tissues that line the body surface of all body organs. They are the special tissue cells which are responsible for triggering immune response and allergic reactions in body upon allergen or pollen contact. Allergic reactions caused after exposure of antigens that lead to degranulation of IgE primed mast cells is known as provocative dose.

The dose of allergen which causes release of antimicrobial cytotoxic substances from secretory vesicles known as granules of mast cell involved in immune response is known as degranulation dose.

3. Basophils: Basophils are responsible for allergic and inflammation reactions that target large invading eukaryotic pathogenic species. Basophils are accumulated to the site of allergic reaction when an allergen is encountered in the body.

4. Allergic mediators: The subsequent exposure rapidly release mediators like leukotrienes, prostaglandins and histamine and occurs within seconds. Therefore type I hypersensitivity is termed as “immediate reaction”. Histamine is the fastest acting mediator which helps in elevation of inflammatory symptom, leukotrienes are released from mast cells in response to allergen exposure and cause inflammation response, and prostaglandins are responsible for increased pain sensitivity and vascular permeability in allergic reactions.

Conclusion

The sensitizing dose of pollen triggers IgE production that binds to mast cells. IgE primed mast cell on encountering pollen again (provocative dose), causes binding of allergen with IgE primed on mast cell leading to degranulation. Degranulation of mast cells results in distribution of allergic mediators in blood that cause inflammatory responses.