Pathophysiology of Alleric Rhinitis

                               Allergic Rhinitis

Allergic rhinitis is inflammation of the nasal mucous membrane caused by exposure to inhaled allergenic materials that elicit a specific immunologic response mediated by immunoglobulin E (IgE).

Types of allergic rhinitis

Seasonal (hay fever)
      It occurs in response to specific allergens (pollen from trees, grasses, and weeds) present at predictable times of the year (spring and/or fall blooming seasons) and typically causes more acute symptoms.

Perennial (intermittent or persistent)
      It occurs year round in response to non seasonal allergens (e.g., dust mites, animal dander, molds) and usually causes more subtle, chronic symptoms.

Many patients have a combination of both types, with symptoms year-round and seasonal exacerbations.

 PATHOPHYSIOLOGY

The initial reaction occurs when airborne allergens enter the nose during inhalation and are processed by lymphocytes, which produce antigen specific IgE, thereby sensitizing genetically predisposed hosts to those agents. On nasal re-exposure, IgE bound to mast cells interacts with airborne allergens, triggering release of inflammatory mediators.

There can be two types of reaction on release of inflammatory mediators:

• Immediate Reaction

An immediate reaction occurs within seconds to minutes, resulting in the rapid release of preformed mediators and newly generated mediators from the arachidonic acid cascade. Mediators of immediate hypersensitivity include histamine, leukotrienes, prostaglandin, tryptase, and kinins. These mediators cause vasodilation, increased vascular permeability, and production of nasal secretions. Histamine produces rhinorrhea, itching, sneezing, and nasal obstruction.

• Late Phase reaction

From 4 to 8 hours after the initial exposure to an allergen, a late-phase reaction may occur, which is thought to be due to cytokines released primarily by mast cells and thymus-derived helper lymphocytes. This inflammatory response likely is responsible for persistent, chronic symptoms including nasal congestion.

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