Text A. Hypersensitivity Reactions.

Allergic reactions, also called hypersensitivity reactions, are reactions of the immune system in which normal body tissue is injured. The mechanisms by which the immune system defends the body and by which a hypersensitivity reaction can injure it are similar. Thus, antibodies, lym­phocytes, and other cells, which are normal pro­tective components of the immune system, are involved in allergic reactions as well as in blood transfusion reactions, autoimmune disease, and organ transplant rejection.

When most people use the term allergic reactions, they are referring to reactions that involve antibodies of the Ig E class. Ig E antibodies bind to special cells, including basophils in the circulation and mast cells in tissues. When Ig E antibodies that are bound to those cells encounter antigens, in this case called allergens, the cells are prompted to release chemicals that injure surrounding tissues. An allergen can be almost anything - a dust particle, plant pollen, a drug, or food - that acts as an antigen to stimulate an immune response.

Sometimes the term atopic diseaseis used to describe a group of often inherited IgE-mediated diseases, such as allergic rhinitis and allergic asthma. Atopic diseases are noted by their tendency to produce Ig E antibodies to harmless in­halants, such as pollens, molds, animal danders, and dust mites. Eczema (atopic dermatitis) is also an atopic disease, although the role of Ig E antibodies in this disorder is less clear. A person with an atopic disease, however, is not at increased risk for developing Ig E antibodies to injected allergens, such as drugs or insect venoms.

Allergic reactions range from mild to severe. Most reactions consist of just the annoyance of watery, itchy eyes and some sneezing. At the extreme, allergic reactions can be life threatening if they involve sudden difficulty in breathing, heart malfunction, and very low blood pressure, leading to shock. This type of reaction, called anaphylaxis, may occur in sensitive people in a variety of situations, such as soon after eating certain foods, taking certain drugs, or being stung by a bee.

Diagnosis

Because each allergic reaction is triggered by a specific allergen, identifying that allergen is the main goal of diagnosis. The allergen may be a seasonal plant or plant product, such as grass or ragweed pollen, or a substance such as cat dan­der, drugs, or foods. The allergen may cause an allergic reaction when it lands on the skin or in the eye, is inhaled, is eaten, or is injected. Often, the allergen can be identified through careful de­tective work by both the doctor and the patient.

Tests may help determine if the symptoms are allergy-related and identify the allergen involved. A blood sample may show many eosinophils, a type of white blood cell that often increases in number during allergic reactions. The radioaller-gosorbent test (RAST) measures blood levels of Ig E antibodies specific to individual allergens, which may help to diagnose an allergic skin re­action, seasonal allergic rhinitis, or allergic asthma.

Skin tests are most useful for identifying partic­ular allergens. For skin testing, dilute solutions made from extracts of trees, grasses, weeds, pol­lens, dust, animal dander, insect venom, foods, and some drugs are individually injected into a person's skin in tiny amounts. If the person is allergic to one or more of these substances, the site at which the relevant solution was injected develops an edematous wheal (a hivelike swelling with surrounding redness) within 15 to 20 minutes. The RAST can be used when a skin test can’t be done or wouldn’t be safe. Both tests are highly specific and accu­rate, although the skin test is generally a bit more accurate and often cheaper and the results are available immediately.

Treatment

Avoiding an allergen is better than trying to treat an allergic reaction. Avoiding the substance may entail stopping use of a particular drug, in­stalling air conditioners with filters, banning a pet from the house, or not consuming a particular kind of food. Sometimes a person allergic to a substance associated with a certain job may have to change jobs. People with strong seasonal aller­gies may consider moving to a region where the allergen doesn't exist.

Other measures involve reducing exposure to an allergen. For instance, someone with an allergy to house dust may remove dust-collecting furni­ture, carpets, and draperies; cover mattresses and pillows. With plastic protectors; dust and wet-mop rooms frequently; use air conditioning to re­duce the high indoor humidity that favors the breeding of dust mites; and install high-efficiency air filters.

Because some allergens, especially airborne al­lergens, can't be avoided, doctors often use meth­ods to block the allergic response and prescribe drugs to relieve the symptoms.

Allergen Immunotherapy

When an allergen can't be avoided, allergen immunotherapy (allergy injections) may provide an alternative solution. With immunotherapy, tiny amounts of the allergen are injected under the skin in gradually increasing doses until a mainte­nance level is reached. This treatment stimulates the body to produce blocking or neutralizing an­tibodies that may act to prevent an allergic reac­tion. Eventually, the blood level of Ig E antibodies, which react with the antigen, also may fall. Im­munotherapy must be carried out carefully, how­ever, because exposure too soon to a high dose of the allergen can itself produce an allergic re­action.

Although many people undergo allergen im­munotherapy, and studies show that it helps, its cost-effectiveness and risk-to-benefit ratio aren't always favorable. Some people and some allergies tend to respond better than others. Immunother­apy is used most often for people allergic to pol­lens, house dust mites, insect venoms, and animal dander. Immunotherapy for people allergic to foods is generally not advised because of the dan­ger of anaphylaxis.

The procedure is most effective when mainte­nance injections are continued throughout the year. Treatments are usually given once a week at first; most people can later get by with mainte­nance injections every 4 to 6 weeks.

Because adverse reactions can follow an im­munotherapy injection, doctors usually insist that the patient remain in the office for at least 20 minutes after an injection. Sneezing, coughing, flushing, tingling sensations, itching, chest tight­ness, wheezing, and hives are all possible symp­toms of an allergic reaction. If mild symptoms occur, medication (typically one of the antihistamines, such as diphenhydramine or chlorpheniramine) may help block the allergic reactions. More severe reactions require an injection of epinephrine (adrenaline).

Antihistamines

Antihistamines are the drugs most commonly used for treating allergies (they're not used for treating asthma). There are two types of histamine receptors in the body: histamine 1 (H1) and histamine 2 (H2). The term antihistaminesgener­ally refers to drugs that block the histamine 1 receptor; stimulation of this receptor by histamine results in injury to target tissues. Histamine 1 blockers shouldn't be confused with drugs that block the histamine 2 receptor (H₂ blockers), which are used to treat peptic ulcers and heartburn.

Many unpleasant but relatively minor effects of an allergic reaction - itchy eyes, runny nose, and itchy skin - are caused by the release of histamine. Other effects of histamine, such as shortness of breath, low blood pressure, and swelling in the throat that can cut off air flow, are more dangerous.

All antihistamines have similar desired effects; they differ mostly in their undesired or adverse effects. Both the desired and the generally undesired effects vary considerably with the specific antihistamine and the person using it. For example, some antihistamines have a greater sedative effect than others, although susceptibility to this effect varies. Sometimes generally undesired effects can be used to advantage. For example, because some antihistamines have what is called anticholinergic effects that dry mucous membranes, they can be used to relieve the runny nose caused by a cold.

Some antihistamines are available without a prescription (over the counter), come in short-acting and extended-release forms, and may be combined with decongestants, which constrict blood vessels and help reduce nasal stuffiness. Other antihistamines require a prescription and a doctor's supervision.

Most antihistamines tend to cause drowsiness. In fact, because of their potent sedative effect, antihistamines are the active ingredient in many over-the-counter sleep aids. Most antihistamines also have strong anticholinergic effects, which can cause confusion, light-headedness, dry mouth, constipation, difficulty with urination, and blurred vision, especially in the elderly. However, most people don't experience adverse effects and can use over-the-counter drugs, which cost much less than the non-sedating prescription antihistamines. Drowsiness and other side effects may also be minimized by starting with a small dose and gradually increasing to a dose that is effective in controlling symptoms. A group of non-sedating antihistamines that also does not cause antiholinergic side effects is now available. This group includes astemizole, cetirizine, loratadine, and terfenadine.

Notes:

mast cell тучные клетки, лаброциты

mold плесень, плесневелый грибок

venom яд

annoyance раздражение

sneezing чихание

radioallergosorbent test (RAST) радиоаллергосорбционный анализ (тест)

dilute разжижать, разбавлять; ослаблять

edematous wheal отечный волдырь

mainte­nance поддержание, сохранение

mainte­nance injections поддерживающие инъекции

flushing гиперемия

chest tight­ness стеснение в груди

hives крапивница

drowsiness сонливость

over-the-counter отпускаемое без рецепта (лекарство)