An allergic reaction occurs when the immune system responds to a foreign substance in an abnormal fashion. The immune system normally protects the body from harmful agents like bacteria and toxins. The body's abnormal response to a harmless substance (called an allergen) is called a hypersensitivity, or an allergic reaction. Generally, these reactions are due to interactions between the allergen and the family of proteins, IgE, resulting in activation of cells in the body called mast cells and basophils.
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