Types of histamine blockers
Types of Histamine Blockers
Histamine is a body chemical that is released in response to digestion in the stomach or as an immune response, such as an allergic reaction. Usually, these types of body reactions are self-limiting and in response to a need. However, blockade of histamine activity can be useful for disorders such as continual seasonal allergies or stomach ulcers. Most types of histamine blockers work on histamine receptors by joining to the receptor so that histamine cannot. There are two common types of histamine receptors and two corresponding classes of histamine-blocking drugs.
First-Generation H1 Histamine Receptor Blockers
H1 histamine receptors are found in smooth muscle cells throughout the body, and they lead to a traditional allergic reaction when histamine is bound to them. The first generation of histamine blockers, antihistamines, was developed to counteract the allergic symptoms. The most common example of a first-generation histamine blocker is diphenhydramine (Benadryl). It and other first-generation antihistamines have a documented side effect of extreme sedation.
Second-Generation H1 Histamine Receptor Blockers
Because of the sedative effect of first-generation histamine blockers, second-generation versions of H1 histamine receptor blockers were developed and marketed in the 1990s. As of 2010, these second-generation histamine receptor blockers are still commonly used in over-the-counter and prescription products and include loratadine (Claritin) and fexofenadine (Allegra). Because of the changed chemical structure of second-generation histamine blockers, less sedation is typically present.
H2 Histamine Receptor Blockers
H2 histamine receptors bind with histamine just as H1 receptors do, but H2 receptors are found in the stomach lining. Stimulating these receptors causes increased digestion and stomach acid secretion. H2 receptor blockers, such as famotidine (Pepcid) and ranitidine (Zantac), are also available over the counter and as prescriptions. These types of histamine blockers are used to counteract excess stomach acid in peptic ulcer disease or gastroesophageal reflux disease (GERD).
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List of Antihistamines
By Claudine LandMay 18, 2015
Antihistamines have a broad range of functions -- they can ease allergy symptoms, alleviate gastrointestinal complaints, prevent motion sickness and aid in sleep, to name just a few. Antihistamines accomplish these tasks by counteracting histamine, an important protein with diverse effects in the body. Histamine acts by binding to other proteins called histamine receptors, which are found on the surface of various types of cells. The receptors are the "locks" that are opened by the "key," histamine. Opening the “lock” initiates a series of reactions within the cells that ultimately lead to histamine’s effects. Antihistamines are commonly classified according to the type of histamine receptor they block.
First Generation H1-Receptor Blockers
The first antihistamines, developed a few decades after the discovery of histamine, block a receptor for histamine known as the H1-receptor. These can be thought of as the "old-fashioned" group of antihistamines. Diphenhydramine (Benadryl) is one of the most common first generation H1-receptor blockers. Other examples include clemastine (Tavist), dimenhydrinate (Dramamine), promethazine (Phenergan) and hydroxyzine (Vistaril, Atarax).
Second Generation H1-Receptor Blockers
Since histamine functions to help keep people awake, the first generation H1-receptor antagonists tend to cause drowsiness. While this is desirable when anithistamines are used as sleep aids, it is a clear disadvantage when these medications are used for other purposes, such as for daytime relief of allergies. In response, researchers developed a new group of H1-receptor antihistamines that do not pass easily into the brain. Because of this, these second generation H1-receptor blockers produce less sleepiness. Common examples of these newer drugs include loratadine (Claritin), fexofenadine (Allegra) and terfenadine (Seldane).
H2-, H3- and H4-Receptor Blockers
Antihistamines have also been developed to block histamine receptors other than H1. H2-receptor blockers -- also known as H2-blockers -- are largely used to reduce acid production in the stomach. These include such common drugs as cimetidine (Tagamet), famotidine (Pepsid, Fluxid), ranitidine (Zantac) and nizatidine (Axid). Several H3-receptor blockers and H4-receptor blockers are under development, but have not yet been approved for use by the US Food and Drug Administration. H3-receptor blockers, such as ciproxifan, may be useful for such conditions as ADHD and Alzheimer's disease.
Related: Types of Histamine Blockers
Histamine Release Inhibitors
Other types of antihistamines work by mechanisms completely different from the histamine receptor blockers. Instead of acting at histamine receptors, histamine release inhibitors prevent the release of histamine from mast cells. These cells release histamine in response to exposure to allergens. Histamine release inhibitors -- also known as mast cell stabilizers -- are primarily used for the prevention of allergy symptoms. Sodium cromoglycate (Intal) is the most common histamine release inhibitor.
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