Antihistamines are one of the commonest medications that are bought over the counter in the pharmacy. Consumers use them to treat allergies and the symptoms of a cold.

The symptoms of an allergy vary from sufferer to sufferer. The symptoms of an allergy may be external or internal. They can affect multiple or single body systems. Hay fever for instance commonly causes itchy eyes and a runny nose, but a cosmetic allergy may cause a widespread rash, swelling and eczema.

The severity of an allergic reaction may vary as well, some reactions are mild and barely noticeable, but a severe reaction, known as anaphylaxis is life threatening. What is common to all allergic reactions is that they are caused by an allergen. An allergen is anything that is recognized by the body and causes a reaction.

Allergic reactions are divided into several types, according to which part of the immune system has been activated by the allergen, but what they all have in common is that they are mediated by the release of histamine from white blood cells. When the body detects a foreign substance white blood cells flood into the area. If the substance is an allergen mast cells release histamines. In the allergic response histamine binds to receptors called H-1 receptors triggering this response.

Histamine causes an immediate increase in the permeability of the blood vessels around the target site, leading to the movement of fluids into the tissues. This causes the puffiness seen on many allergic reactions, and the runny nose and eyes seen with others. When it occurs in the lungs it can be one factor in an allergy induced asthma attack. If the allergen is present throughout the body, for instance something that has been eaten, then a rapid drop in blood pressure can occur leading to collapse. Histamine usually only effects a small part of the body however, such as the eyes or nose.

Histamine causes other effects, such as an increase in gastric acid secretion, which has led to the development of drugs for the treatment of gastric ulcers.

An Italian pharmacologist, Daniel Bovet, working at the Pasteur Institute in Paris in the 1930's discovered that certain chemicals blocked the action of histamine on the body, and by the 1940s the first antihistamines were available.

Antihistamines work by blocking the H-1 receptors and so stopping the allergic response. There are two main groups, the older sedating ones, and the newer non-sedating ones. The sedating ones can cross the blood brain barrier and cause sedation by an action in the brain. Non-sedating ones do not enter the brain. The non-sedating antihistamines, while being better for daytime use, are often not as effective for some symptoms such as itching.

Antihistamines are often removed from the body more quickly than histamine. This means that even if allergen exposure has occurred only once the antihistamine may need to be taken for several days to treat the reaction.

Selection of an appropriate antihistamine usually depends n whether drowsiness is acceptable or even desirable. Many skin reactions cause more itchiness at night and in these cases an older sedating antihistamine may be appropriate. Hay fever sufferers will wish to continue their daily life, and a non-sedating antihistamine will be preferable. The newer antihistamines generally don't react with any medications, but a check should always be made with the pharmacist. Older antihistamines do interact with other medications.

Personal preference and experimentation play a part as the most effective treatment for an allergy will vary from patient to patient and over time.

Learn more about this author, Eve Redstone.
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