AntihistamineAn antihistamine is a chemical which serves to reduce or eliminate effects mediated by histamine, an endogenous chemical of the human body released during allergic reactions, at the histamine receptor. It can be administered locally (through the skin, nose, or eyes) or systemically, based on the type of allergy, and has a wide range of effects depending on the amount and type of antihistamine used. Antihistamines as a group are able to block most of the responses of smooth muscle to histamine. This means that bronchial constriction and vasodilation are prevented. Antihistamines can also block histamine's increase in capillary permeability. This action helps reduce redness and itching.
Histamine's action in allergyIn allergic reactions an antigen (antibody-generating substance) interacts with an IgE antibody molecule and subsequently binds to mast cells and basophils. Once the mast cell-antibody-antigen complex is formed, a complex series of events occurs that eventually leads to the release of histamine from the mast cell. Once released, histamine can react with local or widespread tissues. Some of the effects of histamine include the constriction of the bronchioles, dilation of local blood vessels, increases the force of the heart's contraction, increases the heart rate (speeds up AV depolarization), and more. It is a combination of these effects that is so dangerous in people with severe anaphylaxis, or allergy. Epinephrine, often in the form of an autoinjector (Epi-pen) is a must-have for people with severe allergies. As an aside, it is useful to know that histamine has other effects besides those in allergic responses. Most notably histamine also plays a role in excessive gastric secretion, causing peptic ulcers, GERD, and other such ailments.
Types of antihistaminesThere are two groups of antihistamines:
The histaminergic receptor antagonists are further categorized as follows: First-generation H1-receptor antagonistsThese are the oldest antihistaminergic drugs and are relatively inexpensive and widely available. They are effective in the relief of the symptoms of allergic rhinitis, but are typically moderately to highly potent anticholinergic agents as well. This leads to a variety of potential side-effects, the most common of which is usually drowsiness. Second-generation H1-receptor antagonistsThese are newer drugs that are much more selective for peripheral H1 receptors in preference to the central nervous system histaminergic and cholinergic receptors. This reduces most of the unpleasant side-effects of the older drugs, while still effectively relieving allergy symptoms. H2-receptor antagonistsThese work primarily against the effects of histamine in the gastrointestinal tract. These drugs reduce the secretion of gastric acid, and include ranitidine and famotidine. There is no appreciable anticholinergic effect. H3-receptor antagonistsThese do not yet have a defined clinical use. The H3 receptors are believed to be involved in the regulation and synthesis of histamine. Other agents with antihistaminergic activityIn addition to these drugs, which are used for their antihistaminergic effect, there are also drugs which possess unwanted antihistaminergic activity. One such example is the tricyclic antidepressants. Adverse effectsMost of the adverse effects are associated with the first generation agents. The most common side effect is that of drowsiness (this is the basis of OTC sleep aids). The intake of alcohol can add to this effect. Other first generation agent adverse effects include dizziness, tinnitus, fatigue, blurred vision, urinary retention, cough, dry mouth, and others. Specific H1-antagonistsFirst generation agents
Second generation agents
Categories: Antihistamines | Pharmacologic agents |
|
This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia article. Browse Wikipedia for more information. |