How is an allergic reaction caused by an allergen? Name the drug that can reduce the symptoms of allergy?

Eosinophils constitute 2-4% of the total WBC. They are involved in infections and allergic reactions. The primary allergic reaction is caused as a result of release of mast cells and histamines. 
Eosinophills degranulate on encountering an allergen; thereby, releasing cytokines and other cytotoxic molecules. This in turn activates more mast cells. The allergic reaction is caused as a result of binding of IgE to the specific receptors present on the mast cells.

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