Allergy

People differ widely in their response to allergens, and while some suffer severe allergic (hypersensitivity) reactions to insect bites or particular foods, others suffer no ill effects from exposure to the same substances.

One of the most common allergic disorders, hay fever, is caused by an allergic reaction to inhaled grass pollen, leading to allergic rhinitis -swelling and irritation of the nasal passages and watering of the nose and eyes. Other substances, such as house-dust mites, animal fur, and feathers, may cause a similar reaction in susceptible people. Asthma, another allergic disorder, may result from the action of mediators other than histamine. Other allergic conditions include urticaria (hives) and other rashes (sometimes in response to a medicament), some forms of eczema and dermatitis, and allergic alveolitis (farmer's lung).

Antihistamines

Antihistamines are the most widely used medicaments in the treatment of allergic reactions. They can be subdivided according to chemical structure, each sub-grouping with slightly different actions and characteristics. Their main action is to counter the effects of histamine, one of the chemicals released in the body when there is an allergic reaction.

Histamine is also involved in a number of other body functions, including blood vessel dilation and constriction, the contraction of the muscles of the respiratory and gastrointestinal tracts, and the release of digestive juices in the stomach. The antihistamine medicaments are also known as H 1 blockers because they only block the action of histamine on certain receptors, known as H 1 receptors. Another group of antihistamines, known as H 2 blockers, is used in the treatment of peptic ulcers.

Most antihistamines have a significant anticholinergic action. Advantageous in a variety of conditions, this also accounts for certain undesired side effects.

Antihistamines relieve allergy-related symptoms when it is not possible or practical to prevent exposure to the substance that has provoked the reaction. Their most common use is in the prevention of allergic rhinitis, inflammation of the nose and upper airways resulting from an allergic reaction to a substance such as pollen, house dust, or animal fur. They are more effective when taken before the start of an attack. If they are taken only after an attack has already started, beneficial effects may not be observed or may be delayed.

Antihistamines are not generally effective in asthma caused by similar allergens because the symptoms of this allergic disorder are not solely caused by the action of histamine, but are likely to be the result of more complex mechanisms. When antihistamines fail to provide adequate relief, alternative treatments may be prescribed.

Antihistamines are also useful for relieving the itching, swelling, and redness characteristic of allergic reactions involving the skin -for example, urticaria (hives), infantile eczema, and other forms of dermatitis. Irritation from chickenpox may be reduced by these medicaments. In addition, allergic reactions to insect stings may also be reduced by antihistamines. In such cases the medicament may be taken by mouth or applied topically. Applied as drops, antihistamines also reduce inflammation and irritation of the eyes and eyelids in allergic conjunctivitis.

An antihistamine is often included as an ingredient in cough and cold preparations, when the anticholinergic effect of drying mucus secretions and the sedative effect on the coughing mechanism may be helpful.

Because most antihistamines have a depressant effect on the brain, they are sometimes used to promote sleep, especially when discomfort from itching is disturbing sleep. However, newer antihistamines seem to cause less sedation. Because the depressant effect on the brain also extends to the centers that control nausea and vomiting, antihistamines are therefore often effective for controlling these symptoms.

Occasionally, antihistamines are used to treat fever, rash, and breathing difficulties that may occur in adverse reactions to blood transfusions and allergic reactions to medicaments. Antihistamines block the action of histamine on H 1 receptors. These are found on various body tissues, particularly the small blood vessels in the skin, nose, and eyes. This helps prevent the dilation of the vessels, thus reducing the redness and swelling.

Antihistamines pass from the blood into the brain, where their blocking action on histamine activity produces general sedation and depression of various brain functions, including the vomiting and coughing mechanisms. Antihistamines frequently cause drowsiness and may adversely affect coordination, leading to clumsiness. Some of the newer medicaments have little or no sedative effect. Anticholinergic side effects, including dry mouth, blurred vision, and difficulty passing urine, are common. Most side effects diminish with continued use and can often be helped by an adjustment in dosage or a change to a different medicament.

Because older antihistamines may have a pronounced sedative effect, avoid driving or operating machinery until you know how they affect you. Antihistamines can also increase the sedative effects of alcohol and anti-anxiety medicaments.

In high doses, or in children, some antihistamines can cause excitement. Abnormal heart rhythms have occurred after high doses with some of the newer antihistamines, such as terfenadine, or when some antifungal agents or macrolide antibiotics have been taken at the same time or in people with liver disease, electrolyte disturbances, or heart abnormalities as detected by an electrocardiogram. People with these conditions or with glaucoma or prostate trouble should seek medical advice before taking antihistamines because their various medicament actions may make such conditions worse.

Brain and Nervous System
Eyes and Ears
Gastrointestinal Tract
Heart and Circulation
Hormones and Endocrine System
Infections and Infestations
Malignant and Immune Disease
Muscles, and Joints
Reproductive and Urinary Tracts
Respiratory System
Skin