Antihistamines are a class of medications used to counteract the effects of histamine – a chemical released during an allergic reaction. They relieve the symptoms associated with allergies including runny nose, watery eyes, itchiness and an irritated throat. The most commonly used type of allergy medication, antihistamines are available in many different forms and come in both over-the-counter and prescription strengths.

Antihistamines work by blocking the histamines released by body cells (mast cells and basophils). During an allergic reaction, IgE antibodies in the bloodstream trigger mast cells to release powerful chemicals when they come into contact with an allergen. These chemicals are designed to allow the body to fight off the allergen, which the body has mistaken as a threat. Most allergy symptoms are caused by the cellular swelling and release of fluids caused by the histamines and other chemicals.

As histamine flows through the body, it irritates nerve endings, causing itchiness. Histamines in the gastrointestinal tract can cause diarrhea, fluid secretion and cramps. When histamines interact with muscles in the bronchial airways, the airways can tighten and restrict breathing.

Histamines affect cells by interacting with the receptors on the cell’s surface. Antihistamines have a molecular structure that resembles that of histamine. They therefore block the cell receptors that would usually accept histamines. This essentially makes the histamine molecules into ships with nowhere to dock. With the effects of the histamines interrupted, the severity of allergy symptoms is reduced.

Since antihistamines work best when they can move in and block histamines before they arrive at a cell, it is useful to take an antihistamine treatment before symptoms appear. Older types of antihistamine need about 30 minutes to take effect. Newer, nonsedating antihistamines may need an hour to reach full effect, though some may require several days of use to achieve full effect. It is important to follow the physician's dosage recommendation. A physician might prescribe antihistamines to be taken a week or two before the beginning of hay fever season, for instance, and to continue taking the medication even if symptoms are not present.

A physician can recommend different over-the-counter or prescription antihistamines based on a patient's symptoms. For instance, if symptoms are primarily nasal congestion, then a nasal antihistamine may be recommended. Certain antihistamines may cause certain side effects in some people, so a patient may need to try different brands.

There are two types of antihistamine receptors and medications may be targeted toward blocking one or the other:

• Histamine 1 (H1) receptors. These are located in many body tissues, including the capillaries (small blood vessels). They are involved with most allergic rhinitis (hay fever) reactions. Medications designed to block these receptors are called H1 blockers and are the most commonly used antihistamines.
  
  
• Histamine 2 (H2) receptors. H2 receptors are located in the lining of the stomach. Medications designed to block these receptors are called H2 blockers. H2 blockers were originally developed to treat stomach ulcers, though they have recently been used in combination treatments with H1 blockers to treat hives.

It is important to note that antihistamines can cause falsely negative allergy skin test results. Therefore, patients will need to stop taking the drug some time before the test. The time needed to clear antihistamines from the body before testing should be discussed with a physician.