Table of Contents
Definition of Antihistamine Medications
Antihistamine medications are medications that block the action of histamine, a chemical that acts on the blood vessels during an immune response to allow fluid to flood the tissues. The resulting inflammation is part of the body’s means of delivering infection-fighting agents to the site (such as T-lymphocytes, cytokines, and antibodies). However, this response is exaggerated in a hypersensitivity reaction, during which histamine secretion is excessive or continues after the immune response has neutralized the triggering antigen.
Large, granulated leukocytes (white blood cells) called mast cells produce, store, and release histamine (as well as serotonin and other chemicals). Mast cells reside in the mucous membranes of the respiratory tract (nasal passages, nasal sinuses, trachea, and bronchi) and the gastrointestinal tract (primarily the stomach). Mast cells also store and release serotonin. Histamine is primarily responsible for the inflammatory changes that result in hypersensitivity reaction (allergic reaction).
Stabilizing Mast Cells to Prevent Histamine Release
A different therapeutic approach to managing the histamine cascade in hypersensitivity reaction is to regulate histamine release at the level of the mast cells, which is most effective as a treatment for allergic asthma (hypersensitivity reaction of the airways). drugs called mast-cell stabilizers work by preventing the granules in mast cells from releasing histamine (called degranulation) as part of the immune response. Mast-cell stabilizers to treat allergic asthma include cromolyn and nedocromil.
How These Medications Work
Antihistamines work by blocking the ability of histamine to bind with histamine receptors on the surfaces of cell membranes. Antihistamines target the two types of histamine receptors involved with the immune response: H1 and H3. H1 receptors regulate arteriole dilation and capillary permeability. Histamine causes dilation of the arterioles, the body’s tiniest arteries, to increase blood flow to the tissues and increases the flow of plasma out from the capillaries into the interstitial spaces, to flood the tissues with antibodies, cytokines, prostaglandins, and other molecules essential to the immune response.
When taken at the onset of symptoms (while histamine release is still taking place) or prophylactically (to prevent histamine release, such as to treat seasonal allergies), antihistamine medications relieve the common symptoms of allergy such as itching and sneezing. However, antihistamines cannot reverse the effects of histamine release after they occur or reduce inflammation that has already developed. Antihistamine medications to treat hypersensitivity reaction are available in oral, topical, inhalation, and injection preparations. Doctors may also prescribe antihistamines to relieve nausea, particularly that of motion sickness, and mild anxiety.
First-generation antihistamines, long the mainstay of treatment for allergies, are nonselective. They block both H1 and H3 receptors. H1 blocking subdues symptoms of hypersensitivity reaction. H3 receptors signal brain neurotransmitters in the hypothalamus that regulate alertness and the nau sea center. Antihistamine medications with H3 blocking capability thus cause drowsiness and relieve nausea as well. Second-generation antihistamine medications are selective.
They block primarily H1 receptors and have little effect on H3 receptors; thus they do not generally cause drowsiness and provide little or no relief of nausea. Third-generation antihistamine medications derive from second-generation antihistamines and are purported to have fewer side effects and adverse reactions though functionally are no different. Many antihistamine medications are available in the United States as over-the-counter (otc) drugs, sometimes in combination with a decongestant or other ingredients. Manufacturers often market OTC antihistamines as allergy-relief products. Other antihistamines require a doctor’s prescription.
|First-Generation (Nonselective) Antihistamines|
|Second-Generation (Selective H1) Antihistamines|
|Third-Generation (Selective H1) Antihistamines|
Doctors prescribe or recommend antihistamine medications to treat allergic rhinitis, allergic conjunctivitis, and allergic dermatitis. Most of the nonselective antihistamines cause significant drowsiness; doctors prescribe or recommend them for intermittent insomnia (difficulty sleeping). Meclizine is an H3 receptor antihistamine effective for nausea and vomiting, especially that associated with motion sickness. Meclizine has little effect on H1 receptors, however, so does not influence the immune response or relieve symptoms of hypersensitivity reaction.
Some antihistamine medications have other therapeutic applications, such as
- anxiety: hydroxyzine
- sedative and sleep aid: diphenhydramine, doxylamine, hydroxyzine
- nausea and vomiting: dimenhydrinate, diphenhydramine, hydroxyzine
- vertigo: dimenhydrinate, diphenhydramine
- early Parkinson’s disease: diphenhydramine
Risks and Side Effects
In general antihistamine medications cause few side effects other than drowsiness, although can raise blood pressure. People who take other medications to treat chronic health conditions should check with their doctors before taking antihistamines, as antihistamines can exacerbate symptoms or interfere with the actions of other drugs.
See also ANTIBODY; ARTERY; CORTICOSTEROID MEDICATIONS; CAPILLARY BEDS; GENERALIZED ANXIETY DISORDER (GAD); H2 ANTAGONIST (BLOCKER) MEDICATIONS; LIVING WITH ALLERGIES; LYMPHOCYTE; NEUROTRANSMITTER; PROTON PUMP INHIBITOR MEDICATIONS; SNEEZE.
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