Allergy

Introduction

Allergy is a condition characterised by a level of sensitivity greater than normal to a specific substance or group of substances. These substances, called allergens, trigger a response in susceptible individuals. They can enter the body through various routes like inhalation, ingestion, injection, and external skin contact. They react with antibodies in a susceptible person, causing the release of histamine (a chemical mediator in allergic reactions) and other chemical substances that cause various symptoms, thus creating a chain response known as the allergic response.





Cause and Pathogenesis

Allergic reactions may be caused by a multitude of factors. These include the most common allergy 'triggers' like pollen grains, dust, moulds and foodstuffs. Other triggers include animal proteins from hair, and fur and substances that cause skin allergic reactions such as certain oils found in plants, drugs, cosmetics, chemicals etc.

The allergies can be classified as seasonal allergies and perennial allergies.

Seasonal Allergies

Seasonal allergies occur during certain seasons of the year. They are caused by inhaling microscopic particles in the air and substances like pollen released by plants, grasses and weeds. It can lead to a sustained allergic response.

Perennial Allergies

Perennial allergies occur throughout the year. They occur due to mould, fungi and dust found indoors. Perennial Allergies can be caused due to fur from cats and dogs and other pets.

An exaggerated response to specific food allergens is termed food allergy or hypersensitivity and such foods include milk, eggs, chocolate, nuts, corn, pork and beef. Histamine and its related substances are responsible for mediating the allergic response and causing the characteristic symptoms of allergy.





Symptoms and Signs

Breathing problems and skin irritations are the common allergic responses. Sneezing, running nose, watery eyes, wheezing, skin redness, and skin itching and rashes are common symptoms. Allergic Rhinitis is a term that is used to describe a thin, watery trickle from the nose accompanied by sneezing, as in common colds. Severe allergies can cause anaphylactic shock, a life threatening reaction to some allergens where there may be a sudden and dramatic drop in blood pressure. Anaphylactic shock may also cause closure of air passages and result in death. Anaphylactic reactions may be caused by bee stings, drugs like penicillin and also some types of food substances. When an individual's allergies are triggered, there is swelling of the nasal linings, which results in sinus blockage and infection.





Investigations and Diagnosis

A family history of allergy and allergy-related diseases is important in the assessment of allergies. Reactions occurring with regularity at certain times each year may be due to seasonal allergies, while those that manifest throughout the year are perennial. If the symptoms improve with anti-allergy drugs, then an allergic component to the problems is believed to be present.

There are also a variety of tests that help in the diagnosis of allergies. One method of testing is In-Vivo Testing. The basic principle in this method is to demonstrate an actual allergic reaction in the patient by using micro doses of the suspected allergen. Various allergens are injected into the skin by a scratch, a prick, or an injection, and the skin is monitored for a reaction. A positive response can be noted when significant redness or swelling occurs at the site of the test. While these tests can accurately identify actual allergic sensitivities, there is the risk of triggering a full-blown allergic response. Another requirement of In-Vivo Testing is that the patient must stay off anti-allergy drugs for several days prior to testing so as not to negate the response. Additionally, this test involves the timely and sometimes painful process of injecting numerous allergens, usually on the back or the arms. Skin tests are difficult to perform on infants and young children. Allergic individuals who have skin rashes or hypersensitive skin, which might interfere with test results, are not suitable for skin testing.

Another method is In-Vitro Testing. It is based on identifying the antibody proteins (chemicals produced in the body due to an immune response) produced by the individual during an allergic reaction. A blood sample is collected from the patient, and the amount of specific antibodies present is measured (there are a variety of antibodies, each of which reacts to a specific allergen). The Radio-Allergo-Sorbent Test (RAST) requires only a sample of the patient's blood. Blood serum can then be tested against common allergens. However, RAST is useful in infants and young children unable to undergo the In-Vivo Test, and in individuals with extensive skin rashes or skin hypersensitivity where injection may stimulate an over-response.

In Nasal Smear Testing, a sample of nasal secretions is collected and analysed in a laboratory. Substances called Eosinophils (a type of cell present in the body) are produced during an allergic reaction, and these are observed under a microscope in the nasal specimens obtained. This test does not identify the specific substances that cause the allergy but indicates if the nasal problems are caused due to allergy.





Treatment and Prognosis

Avoiding the causative factor is essential to prevent allergies. Since the allergic reaction is due to histamine and its related substances that trigger the full-blown allergic reaction, drugs called Antihistaminics are used to treat allergies. They block the activity of histamine and relieve symptoms. Their most common side effect is drowsiness. Some antihistamines are non-sedating, and do not cause drowsiness. Antihistaminics in the form of nasal sprays work relatively rapidly once sprayed into the nose. Antihistaminic sprays avoid some of the unpleasant side effects of oral antihistamines. Antihistamine-Decongestant combinations are useful in allergic patients who have a strong degree of nasal congestion. While the antihistamine component minimises sneezing and dripping, the decongestant contracts the lining of the nose, resulting in improvement in breathing. The decongestant component tends to cause an increasing wakefulness, which counteracts the sedating aspects of antihistamines.

Steroids are the most potent drugs for allergies. They help by decreasing the inflammation seen in the allergic reaction. Prolonged use of steroids can have serious adverse effects, so they should be taken on a limited, short-term basis to provide relief. Steroid sprays are relatively safer than oral steroids. Other intranasal sprays used include cromolyn sodium that acts by preventing the release of histamine. It is most effective if used prior to allergen exposure. Another spray called Ipratropium acts directly on the glands of the nasal lining to decrease the watery trickle that occurs with allergies and other conditions. Anaphylaxis should be treated by immediate administration of Epinephrine.

If environmental controls and medications have failed to provide adequate relief, allergy shots (Allergen Immunotherapy or Desensitization) can be used. Shots are more useful for year-round allergies rather than those lasting a short period each year. The basic principle is to inject increasing doses of allergens into the allergic patient to sensitise him. The shot is made up of an extract based on results of the skin test or a RAST. Usually, shots are given once to twice every week, with increased dosage each week. Once a maintenance level is reached, the shots can be given at reduced frequency. Significant improvement should be seen within three to six months of initiating Immunotherapy. If successful, the treatment should be given for two to five years.





Prevention


The best way to avoid symptoms of allergy is prevention. This means avoiding exposure to agents that trigger the allergic response. Being aware of common allergens like pollen, dust, fur, drugs, specific foods etc, and avoiding them is absolutely essential in preventing an allergic reaction. Patients, who have suffered from an anaphylactic reaction in the past should carry an injectable shot of a drug called Epinephrine that can be administered in an emergency. Prompt treatment of severe allergic reactions, especially anaphylactic reactions, is essential.