The Immune System
To understand what an antihistamine does, a person must understand the basic functions of the immune system and the role of histamine. The body has many layers of protection between its internal environment and the outside world. It has passive physical barriers, such as skin and cell membranes, as well as active defenses such as those the immune system provide. Mast cells are a part of the immune system that act as first responders if the physical barriers are breached; these cells are generally found in the areas around blood vessels and nerves.
When potentially dangerous foreign substances (antigens) enter the body, they are captured by small proteins in the blood and then bound to receptors on the surface of the mast cells. When something stimulates these receptors, a chemical cascade begins that results in release of a variety of substances, including histamine, into the interstitial space. These substances not only mediate the inflammatory response, but also serve to attract components of the immune system, such as white blood cells, that are designed to destroy invaders to the area.
The Role of Histamine
Histamine has a special role in the immune response. Upon release, histamine binds to specific receptors on cells in surrounding tissues, including nearby nerves and capillaries. These cells and capillaries respond by becoming "leaky," resulting in a local buildup of fluid. The resulting swelling serves to isolate the area and keep the antigen localized until the mobile components of the immune system can arrive at the scene, at which time the extra fluid also assists the immune system in clearing the invaders by allowing better access.
The Downside of Protection
As effective as this type of response is in containing infection, the response itself can make life uncomfortable. In the nose, for example, the swelling of the tissues creates the sensation of a stuffy nose, and the extra fluid tends to leak out and down as a runny nose. Histamine stimulation of the nerves also has unpleasant effects; it can lead to sneezing in the nose, while in the skin it can lead to the itching associated with many insect bites. In some cases, inflammatory reactions can become severe, even life-threatening, such as when tissues of the throat become so swollen that it becomes impossible to breathe.
The Role of Antihistamines
Antihistamines are a class of substances that are very similar to histamine, but they do not have the same effect on the tissues. When present in the body, antihistamines bind to histamine receptors on nerves, capillaries and other tissues interfering with histamine's ability to bind there. Thus, although the histamine is inevitably released by the mast cells, it does not cause the membranes and capillaries to become leaky, thereby reducing the amount of fluid and swelling that occurs. In this way, the antihistamine reduces swelling and relieves the runny nose. Similarly, by blocking histamine's effect on nerves, antihistamines can relieve sneezing as well as the itch associated with skin reactions.
How to To Take Antihistamines
Antihistamines are generally administered to relieve the unpleasant or potentially dangerous side effects of a histamine reaction. For most individuals, antihistamines are delivered either orally or topically; in selected cases, they can be delivered intravenously. Antihistamines are most effective when given as soon as the reaction begins. Most antihistamine creams should only be reapplied every four hours or so. More frequent exposure to these preparations can result in a rash. Oral antihistamines vary widely in dosing and side effects; therefore, a person should carefully read the packaging before taking or administering antihistamines. The old standard antihistamines such as such as diphenhydramine (Benadryl) or pseudoephedrine (Sudafed) need to be taken every four to six hours and can cause drowsiness. Newer medications, such as loratadine (Claritin) and cetirizine (Zyrtec) can be effective for as long as 24 hours.
