Allergy:

An allergy is an over-reaction of the immune system to a normally harmless substance called an allergen.

Common allergens include pollen, animal dander, down feathers, mites, chemicals, and a variety of foods.

Some allergies primarily cause respiratory symptoms like sneezing, stuffy and/or runny nose. Others can cause diverse symptoms like a rash or hives, lip or tounge swelling, wheezing, and shorthness of breath or decreased blood pressure.

Allergies can be acute, chronic or seasonal.

Allergic rhinitis is a common type of allergy.

On first exposure, the inhaled allergen enters the mucous membrane lining the nasal passages where it is taken up by the antigen-presenting cell which presents it to the T cells.

These T cells activate the B cells to release substances called IgE antibodies against the allergen.

These IgE antibodies sit on the surface of the mast cells.

The mast cells have granules containing chemical mediators like histamine and prostaglandins etc.

On exposure the allergen binds to the IgE antibodies present on the mast cells, crosslinking them.

This results in the release of histamine, prostaglandins and other mediators into the surrounding tissue.

These mediators cause dialation of the surrounding blood vessels and increase their permeability.

This results in the nasal stuffiness, sneezing and mucous discharge of allergic rhinitis. If more severe, a generalized reaction with other, more severe symptoms may occur.

Antihistamines are the medications most commonly used to diminish allergic symptoms. Antihistamines work by blocking the action of histamines at its receptors and thus decreasing the body's reaction to allergen.