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How does histamine release from mast cells cause allergy symptoms?
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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.
When an allergic reaction occurs, the immune system identifies harmless substances as threats, triggering mast cells to release histamine in excessive amounts. This process, known as degranulation, occurs when antibodies bound to the mast cells encounter these foreign substances, or antigens.12
Once released, histamine binds to specific receptors on nearby cells, leading to several physiological effects that manifest as allergy symptoms:13
- Vascular changes: Histamine causes blood vessels to dilate and become more permeable, allowing fluid and immune cells to leak into surrounding tissues, which results in swelling, redness, and rashes.13
- Smooth muscle contraction: It stimulates the contraction of smooth muscles, particularly in the respiratory system, which can lead to the constriction of bronchi.123
- Secretory responses: It increases mucus production, contributing to symptoms like a runny nose and watery eyes.12
These reactions are the body's attempt to address what it perceives as an injury or infection, but in the context of an allergy, they produce the distressing symptoms associated with hypersensitivity.13