Allergic Reactions

An allergy is a hypersensitive immune response to a normally harmless substance. The first process necessary for establishing an allergy is sensitization.

For example, individuals who are allergic to grass pollen have undergone sensitization. Their immune cells lining the respiratory passage take up and degrade the allergens into fragments.

These immune cells are called antigen presenting cells, or APCs because they display the degraded allergen fragments on their surface.

The APCs activate another encountered cell, type two T helper cells, abbreviated as Th2.

The activated Th2 then release chemical signals that cause another type of immune cell, B cells, to develop into antibody producing plasma cells which make immunoglobulin E, or IgE, a class of antibodies.

In this case, exposure to grass pollen will produce an IgE specific to this pollen, causing an allergic reaction distinct to grass pollen and no other type.

Once created, IgE binds to the surface of specialized cells called mast cells, found in tissues like the mucous membranes and skin.

Following sensitization, an allergic reaction occurs each time an individual encounters the allergen to which they are allergic after the allergen binds to its specific IgE on mast cells.

For instance, when the grass pollen allergen binds to its IgE, it causes the release of allergy symptom causing chemicals, such as leukotrienes and histamines that lead to sneezing, runny noses, and nasal passage inflammation.

Allergen exposure also causes mast cells and Th2 to release chemical signals that recruit and activate other inflammation inducing immune cells such as eosinophils and basophils to further amplify symptoms.