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Histamine and Histamine Receptors Definition
A chemical that acts as an IMMUNE RESPONSE mediator. Large, granulated leukocytes called mast cells, which reside in the mucous membrane lining of the respiratory and gastrointestinal tracts, store histamine in their granules and release it during the immune response. Mast cells are most abundant in the nasal passages (including the SINUSES), the TRACHEA, and the bronchi.
Histamine receptors on the surfaces of cell membranes determine how histamine affects the cell. ANTIHISTAMINE MEDICATIONS, the cornerstone of treatment for type I HYPERSENSITIVITY REACTION (allergic reaction), work by blocking histamine receptors.
Types of Histamine Receptors
Though there is only one form of histamine, its release can activate any of three types of histamine receptors: H1, H2, and H3. CYTOKINES, PROSTAGLANDINS, and other biochemical messengers determine histamine release and what histamine binding will occur. Each histamine receptor regulates a different response:
- H1 receptors are on cell membrane surfaces of arteriole and capillary cells. H1 binding causes the arterioles to dilate (open) and the capillaries to increase the permeability of their walls. The effect of these actions is to allow additional fluid to seep from these blood vessels into the interstitial spaces (spaces between cells). The fluid floods the cells with INFECTION-fighting molecules, notably antibodies and cytokines. IMMUNOGLOBULIN E (IgE) binds with antigens and allergens, triggering H1 release. H1 is primarily responsible for type I hypersensitivity reactions such as ALLERGIC RHINITIS and ALLERGIC ASTHMA. Common antihistamine medications that block H1 receptor binding include diphenhydramine, chlorpheniramine, and hydroxyzine.
- H2 receptors are in parietal cells of the STOMACH. H2 binding acts to increase the flow of gastric acid in the stomach. Excessive secretion of histamine binding with H2 receptors is primarily responsible for GASTROESOPHAGEAL REFLUX DISORDER (GERD). Medications to limit histamine secretion and H2 receptor binding include H2 ANTAGONIST (BLOCKER) MEDICATIONS such as cimetidine and ranitidine.
- H3 receptors are neuroreceptors in the CENTRAL NERVOUS SYSTEM with high concentration in the areas of the HYPOTHALAMUS that regulate alertness. H3 binding decreases NEURON (NERVE cell) secretion of histamine, serotonin, acetylcholine, EPINEPHRINE, and NOREPINEPHRINE. The effect is to reduce alertness, which takes place as a natural aspect of the circadian cycle (body’s rhythm of sleep and wakefulness). These neurotransmitters also affect the NAUSEA center. The antihistamine medications doxylamine and hydroxyzine are highly effective H3 receptor blockers.
Doctors typically consider only H1 receptor binding in the context of the immune response and focus primarily on whether its actions to initiate INFLAMMATION are helpful or counterproductive.