sinusitis, acute or chronic inflammation of the mucosal lining of one or more paranasal sinuses (the cavities in the bones that adjoin the nose). Sinusitis commonly accompanies upper respiratory viral infections and in most cases requires no treatment. Purulent (pus-producing) sinusitis can occur, however, requiring treatment with antibiotics. Chronic cases caused by irritants in the environment or by an impaired immune system may require more extended treatment, including surgery.

The origin of acute sinus infection is much like that of ear infection (see otitis media). Normally, the middle ear and the sinuses are sterile, but the adjacent mouth and nose have a varied bacterial flora. Under normal conditions, very small hairs called cilia move mucus along the lining of the nose and respiratory tract, keeping the sinuses clean. When ciliary function is damaged, infection can be established. Following a common cold, a decrease in ciliary function may permit bacteria to remain on the mucous membrane surfaces within the sinuses and to produce a purulent sinusitis. The organisms usually involved are Haemophilus influenzae, Streptococcus pneumoniae, Staphylococcus aureus, Streptococcus pyogenes, and many other penicillin-sensitive anaerobes.

Common symptoms include facial pain, headache, and fever following previous upper respiratory viral illness. On physical examination, persons with sinusitis are usually found to have an elevation in body temperature, nasal discharge, and sinus tenderness. Diagnosis can be confirmed by X-rays of the sinuses and cultures of material obtained from within the sinuses.

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Treatment of acute sinusitis is directed primarily at overcoming the infecting organism by the use of systemic antibiotics such as penicillin and at encouraging drainage of the sinuses by the use of vasoconstricting nose drops and inhalations. If the infection persists, the pus localized in any individual sinus may have to be removed by means of a minor surgical procedure known as lavage, in which the maxillary or sphenoidal sinuses are irrigated with water or a saline solution.

Chronic sinusitis may follow repeated or neglected attacks of acute sinusitis, particularly if there is impaired breathing or drainage due to nasal polyps or obstructed sinus openings. It may also be caused by allergy to agents in the environment, such as fungi or pollen. The symptoms of chronic sinusitis are a tendency to colds, purulent nasal discharge, obstructed breathing, loss of smell, and sometimes headache. Pain is not a feature of chronic sinusitis. If antibiotic therapy or repeated lavage do not alleviate the condition, steroidal medications may be given to relieve swelling and antihistamines to relieve allergic reactions. In severe cases, endoscopic surgery may be necessary to remove obstructions.

The Editors of Encyclopaedia BritannicaThis article was most recently revised and updated by Kara Rogers.

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allergy, hypersensitivity reaction by the body to foreign substances (antigens) that in similar amounts and circumstances are harmless within the bodies of other people.

Antigens that provoke an allergic reaction are called allergens. Typical allergens include pollens, drugs, lints, bacteria, foods, and dyes or chemicals. The immune system contains several mechanisms that normally protect the body against antigens. Prominent among these are the lymphocytes, cells that are specialized to react to specific antigens. There are two kinds of lymphocytes—B cells and T cells. B cells produce antibodies, which are proteins that bind to and destroy or neutralize antigens. T cells do not produce antibodies; instead, they bind directly to an antigen and stimulate an attack on it. Allergic reactions can have immediate or delayed effects, depending on whether the antigen triggers a response by B cells or T cells.

Allergic reactions with immediate effects are the result of antibody-antigen responses (i.e., they are the products of B-cell stimulation). These can be divided into three basic types.

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Type I reactions, which include hay fever, insect venom allergy, and asthma, involve the class of antibodies known as immunoglobulin E (IgE). IgE molecules are bound to mast cells, which are found in loose connective tissue. When enough antigen has bound with the IgE antibodies, the mast cells release granules of histamine and heparin and produce other agents such as leukotrienes. These potent chemicals dilate blood vessels and constrict bronchial air passages. Histamine is responsible for the visible symptoms of an allergic attack, such as running nose, wheezing, and tissue swelling. A severe, often fatal, type I allergic reaction is known as anaphylaxis. The predisposition of a person to type I allergic reactions is genetically determined. The best protection against such allergies is avoidance of the offending substance. Antihistamine drugs are often used to give temporary relief. Another helpful measure is desensitization, in which increasing amounts of the antigen are injected over a period of time until the sufferer no longer experiences an allergic response.

Type II reactions result when antibodies react with antigens that are found on certain “target” cells. The antigens may be natural components of healthy cells, or they may be extrinsic components induced by drugs or infectious microbes. The resultant antigen-antibody complex activates the complement system, a series of potent enzymes that destroy the target cell.

Type III reactions result when a person who has been strongly sensitized to a particular antigen is subsequently exposed to that antigen. In a type III reaction, the antigen-antibody complex becomes deposited on the walls of the small blood vessels. The complex then triggers the complement system, which produces inflammation and vascular damage. Unlike type I reactions, type II and type III reactions are not dependent on a genetic predisposition. Avoidance of known allergens is the best protection against such reactions.

Delayed, or type IV, allergic reactions are caused by the actions of T cells, which take longer to accumulate at the site where the antigen is present than do B-cell antibodies. The allergic responses appear 12 to 24 hours or more after exposure to an appropriate antigen. A common delayed allergic reaction is contact dermatitis, a skin disorder. The rejection of transplanted organs is also mediated by T cells and thus may be considered a delayed allergic response.

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The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Kara Rogers.