Medical treatment of seasonal allergies is based on blocking the mechanisms that produce symptoms. Production of Type 1 hypersensitivity symptoms depends upon a cascade of biochemical events in the body.
First, IgE antibodies attach to the surface of cells that will later produce the allergic response. These are called “allergy effector cells.” The most important effector cells are mast cells and eosinophils. When mast cells encounter an allergen with a specific affinity for the IgE on their surface, they release an array of chemicals called “mediators” that cause inflammation. Most of the symptoms of seasonal allergy result from the release of mast cell mediators. Sodium cromoglycate, inhaled into the nose or lungs, may prevent release of mast cell mediators. Most drugs used for seasonal allergy block the effects of mediators.
Drugs That Block Mast Cell Mediators
There are more than 200 mast cell mediators. The best-known and the drugs that block them are:
Histamine causes the typical symptoms of acute allergies by dilating blood vessels to produce redness and heat. Histamine makes blood vessels leaky so that plasma seeps out into the surrounding tissues, causing swelling.
Antihistamines are the standard first-line therapy for symptoms of seasonal allergy. They are taken as pills or as nasal sprays. Antihistamine side effects include drowsiness and dry mouth.
Serotonin causes itching and may also cause abdominal cramps and diarrhea. When antihistamines alone do not relieve allergic itching, a drug like cyproheptadine, which blocks serotonin, may be added. Drowsiness is its main side effect.
Prostaglandin D2 (PGD2) causes constriction of bronchial tubes and plays a key role in the wheezing associated with asthma. It also dilates blood vessels to cause flushing of skin or redness of eyes. Some of the eye drops used for ocular allergy block the synthesis of PGD2.
Leukotrienes (LTs) increase mucus secretion and make bronchial tubes constrict. They contribute to the misery of asthma and hay fever. LT antagonists, such as montelukast, are prescription drugs that can relieve symptoms of seasonal allergic rhinitis and asthma. Depression is an unexpected side effect, especially reported among teenagers.
Other mast cell mediators recruit eosinophils into the inflamed tissues. Eosinophils secrete enzymes that can damage tissues. The medications that most effectively block eosinophils are steroids.
Steroids are drugs related to the anti-inflammatory hormone hydrocortisone, which is produced by the adrenal glands. They have very powerful anti-allergic effects and many serious side effects, which limit their use. Pills or injections of steroids are only used for emergency treatment. Even with short-term use, they can cause mood changes, fluid retention, weight gain, stomach ulcers, increase of blood pressure and blood sugar and increased susceptibility to infection.
Nasal sprays, inhalers and creams containing synthetic steroids are commonly used for relief of symptoms and often used for extended periods of time. Side effects of these occur because of systemic absorption from inflamed tissues or from local toxicity. Steroids interfere with wound healing and can cause atrophy of tissues so that nasal steroids may cause nose bleeds and steroid creams may cause thinning of the skin. Systemic absorption of nasal steroids has been associated with growth retardation in children and with cataract formation, upon prolonged use.
Allergy Shots (Immunotherapy)
Injections for seasonal allergies are intended to stimulate the production of “blocking antibodies.” These bind to the specific allergen and prevent it from binding to the IgE attached to mast cells. The classic technique has involved the injection of increasing doses of allergen under the skin weekly for months or years. Because of the risks associated with this technique, which include local and systemic allergic reactions, novel forms of immunotherapy have been developed.
Sublingual immunotherapy (SLIT) has recently been approved in the U.S. as an alternative way to treat allergies without injections. An allergist gives patients small doses of an allergen under the tongue. SLIT has been widely accepted in Europe, South America, Asia and Australia for many years. Research indicates that SLIT works through multiple mechanisms, including induction of immune cells called regulatory T cells that blunt the allergic response rather than just blocking its effects.