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Why is anaphylaxis considered a severe type I allergic reaction?

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antihistamines, including Benadryl — antihistamine Antihistamines such as Benadryl (diphenhydramine) and Claritin (loratadine) can be purchased without prescriptions. Both of those agents act by blocking H₁ receptors. However, whereas Benadryl binds to those receptors in the central nervous system (CNS), causing drowsiness, Claritin does not readily enter the CNS and thus does not normally cause drowsiness.

Anaphylaxis is considered a severe type I allergic reaction because it is a systemic, potentially fatal response that affects the entire body, rather than being localized to a specific region like the respiratory tract.1

As a type I hypersensitivity reaction, it is mediated by immunoglobulin E (IgE) antibodies. When a sensitized individual is exposed to an antigen, these antibodies trigger mast cells and basophils to release massive, inordinate amounts of inflammatory chemicals, primarily histamine.123

This widespread release causes severe physiological effects throughout the body, including:
* Vascular dilation and permeability: Leading to an abrupt, dangerous drop in blood pressure and widespread swelling.13
* Smooth muscle constriction: Causing severe difficulty in breathing due to bronchial spasms.13

While antihistamines are used as part of the treatment to counteract histamine's effects, they are secondary to epinephrine, which is required to rapidly reverse the life-threatening systemic collapse characteristic of anaphylaxis.1