hypersensitivity pneumonitis

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Hypersensitivity Pneumonitis

 

Definition

Hypersensitivity pneumonitis refers to an inflammation of the lungs caused by repeated breathing in of a foreign substance, such an organic dust, a fungus, or a mold. The body's immune system reacts to these substances, called antigens, by forming antibodies, molecules that attack the invading antigen and try to destroy it. The combination of antigen and antibody produces acute inflammation, or pneumonitis (a hypersensitivity reaction), which later can develop into chronic lung disease that impairs the lungs' ability to take oxygen from the air and eliminate carbon dioxide.

Description

Hypersensitivity pneumonitis (HP) is sometimes called "allergic alveolitis." "Allergic" refers to the antigen-antibody reaction, and "alveolitis" means an inflammation of the tiny air sacs in the lungs where oxygen and CO2 are exchanged, the alveoli. It also is known as "extrinsic" allergic alveolitis, meaning that the antigen that sets up the allergic reaction (also called an allergen) comes from the outside. Most of the antigens that cause this disease come from plant or animal proteins or microorganisms, and many of those affected are exposed either at work or in the course of some hobby or other activity. The first known type of HP, farmer's lung, is caused by antigens from tiny microorganisms living on moldy hay. An example of disease connected with a hobby is pigeon breeder's lung, caused by inhaling protein material from bird droppings or feathers. After a time, very little of the allergenic material is needed to set off a reaction in the lungs.
Roughly one in every 10,000 persons develops some form of HP. A mysterious aspect of this condition is that, even though many persons may be exposed to a particular antigen, only a small number of them will develop the disease. Genetic differences may determine who becomes ill; this remains unclear. Probably between 5% and 15% of all persons who are regularly exposed to organic materials develop HP. Most of those who do get it do not smoke (smoking may create the type of cells that take up antigens and neutralize them). The amount of antigen is an important factor in whether HP will develop and what form it will take. Sudden heavy exposure can produce symptoms in a matter of hours, whereas mild but frequent exposures tend to produce a long-lasting, "smoldering" illness. HP may be more likely to develop in persons exposed to polluted air or industrial fumes.
Typical changes occur in the lungs of persons with HP. In the acute stage, large numbers of inflammatory cells are found throughout the lungs and the air sacs may be filled by a thick fluid mixed with these cells. In the subacute stage, disease extends into the small breathing tubes, or bronchioles, and the inflammatory cells collect into tiny granules called granulomas. Finally, in the chronic stage of HP, the previously inflamed parts of the lungs become scarred and unable to function, as in pulmonary fibrosis.

Causes and symptoms

A number of different types of HP are known, since a wide range of allergens may produce an allergic reaction in the lungs. Many of them produce similar symptoms and abnormal physical findings, but some have their own typical features. Some of the more common forms are:
  • Farmer's lung. Can affect any farmer who works with wet hay or other moldy dust. Small farmers who have to directly thresh and handle their hay are most at risk, as are those living in cold and humid areas where damp weather is common.
  • Pigeon breeder's lung. Also called "bird fancier's lung," it is second to farmer's lung as the best known type of HP. A substance has been found in pigeon droppings that may cause the allergic reaction, but there may be more than one such substance. Besides pigeons, the disorder may follow exposure to ducks, geese, pheasants, and even canaries. Parakeets produce an especially severe form of disease. Most patients are middle-aged women, who usually care for birds either at home or on bird breeding farms.
  • Bagassosis. Caused by bagasse, a substance produced when juice is extracted from sugar cane and is used in making paper and explosives. A fungus is probably responsible. Young and middle-aged men who work in the sugar industry are at risk.
  • Byssinosis. A similar condition affecting workers who inhale dust from cotton, flax, or hemp.
  • Humidifier lung. An acute form of HP caused by inhaling actinomycyetes, the same organisms that cause farmer's lung, which grow in contaminated humidifier vents, air conditioners, heating systems, and even saunas.
  • Other antigens. HP has been seen in persons working with detergents, silicone, mushrooms, cheese, wood dust, maple bark, coffee, and furs.
In the acute stage, patients with HP begin coughing, develop fever, and note tightness in the chest as well as extreme tiredness and aching, four to eight hours after the most recent exposure. Most patients are well aware of the connection between their work (or an activity) and their symptoms. After a time, patients may have trouble breathing. They also may lose their appetite, lose weight, and generally feel ill. Finally, in the chronic stage, the patient will have increasing trouble breathing and may sometimes wheeze. With advanced disease, the skin may appear bluish (because too little oxygen is getting into the blood). When the physician listens to the patient's chest with a stethoscope, there may be crackling sounds or loud wheezing. In the late stages, club-shaped fingertips are a sign that the patient has not been getting enough oxygen for an extended period of time.

Diagnosis

No single test can make a definite diagnosis of HP. The key is to relate some specific exposure or activity to episodes of symptoms. The chest x ray may be normal in the acute stage, but later may show a hazy appearance that looks like "ground glass." There may be linear or rounded shadows in the central parts of the lungs. Studies of lung function in the acute stage typically show abnormally small lung volume. The ability to breathe at a fast rate is impaired. Blood from an artery typically has a low level of oxygen. Later, when the lungs have begun to scar, the airways (breathing tubes) are obstructed and the rate of air flow is reduced.
Some experts believe that skin testing can help diagnose HP and show which particular antigen is causing the symptoms. Small amounts of several suspect antigens are injected just beneath the surface of the skin, usually on the arm or back, and the reactions compared to that caused by injecting a harmless salt solution. Another diagnostic test is to place a thin tube into the airways, inject a small amount of fluid, and draw it back up (bronchoalveolar lavage). A very large number of cells called lymphocytes is typical of HP, and mast cells, which are part of the immune system, may also be seen. Rarely, a tissue sample (biopsy) of lung tissue may be taken through a tube placed in the airways and examined under a microscope. Finally, a patient may be "challenged" by actually inhaling a particular antigen in the form of an aerosol and noting whether lung function suddenly becomes worse. This test is usually not necessary.

Treatment

Treatment of HP requires identifying the offending antigen and avoiding further exposure. Although it may sometimes be necessary for a patient to find a totally different type of work, often it is possible to simply perform different duties or switch to a work site where exposure is minimal. In some cases, (like pigeon breeder's lung), wearing a mask can prevent exposure. If acute symptoms are severe, the patient may be treated with a steroid hormone for two to six weeks. This often suppresses the inflammatory response and allows the lungs a chance to recover. In the chronic stage, steroid treatment can delay further damage to the lungs and help preserve their function.

Prognosis

In general, most of the symptoms of HP disappear when the patient is no longer exposed to the causative allergen. The actual chances of complete recovery depend in part on what form of HP is present. Older patients and those exposed repeatedly for long periods after initially developing symptoms tend to have a poorer long-term outlook. The worst outcome is that long repeated episodes of exposure will cause chronic lung inflammation, scar the lungs, and permanently make then unable to properly provide oxygen to the blood. Rarely, a patient will become permanently disabled.

Prevention

It is often not possible to prevent initial episodes of HP, because there is no way of predicting which individuals (such as farmers) will have an allergic reaction to a particular allergen. Once the connection is made between a type of exposure and definite hypersensitivity symptoms, prevention of further episodes is simple as long as further exposure can be avoided.
Exactly how to avoid exposure depends on a person'swork or activities and what he or she is reacting to. People with farmer's lung can dry hay thoroughly before storing it. For pigeon breeder's lung (and many other types of HP), a mask can be worn. In many industrial settings, it is possible to take precautions that will limit the amount of allergen that workers will inhale. If it is not possible to avoid exposure altogether, exposure can be timed and strictly minimized.

Resources

Organizations

American Lung Association. 1740 Broadway, New York, NY 10019. (800) 586-4872. http://www.lungusa.org.
Asthma and Allergy Foundation of America. 1233 20th Street, NW, Suite 402, Washington, DC 20036. (800) 727-8462. http://www.aafa.org.

Key terms

Allergen — An outside substance, such as dust or a mold, that, when inhaled, sets off an allregic (hypersensitivity) reaction in the lungs.
Fibrosis — A result of long-standing inflammatory disease in which normal tissue is replaced by scar tissue that is functionally useless.
Granuloma — A collection of inflammatory cells forming a microscopic lesion, many of which are scattered throughout the lung tissue in patients who have had numerous acute episodes of HP.
Hypersensitivity — After the body's immune system attacks an outside invader (such as organic dust or a fungus) many times, exposure to even a tiny amount of this allergen can provoke a strong inflammatory response.
Pneumonitis — Inflammation of the lung tissues.
Steroid — A natural body substance may be given orally or by injection, and serves to dampen or even halt inflammation anywhere in the body, including the lungs.

pneumonitis

 [noo″mo-ni´tis]
inflammation of the lung; see also pneumonia.
hypersensitivity pneumonitis a respiratory hypersensitivity reaction to repeated inhalation of organic particles, usually in an occupational setting, with onset a few hours after exposure to the allergen. Many different substances are potential causes of the condition; see bagassosis, farmer's lung, and pigeon breeder's lung. Characteristics include fever, fatigue, chills, unproductive cough, tachycardia, and tachypnea; in the chronic form there is interstitial fibrosis with collagenous thickening of the alveolar septa. Called also extrinsic allergic alveolitis.
radiation pneumonitis lung inflammation resulting from radiation exposure, usually radiation therapy, with coughing, dyspnea, and alveolar infiltration of secretions, leading to mild to severe or even fatal fibrosis 6 to 9 months after the exposure.

hy·per·sen·si·tiv·i·ty pneu·mo·ni·tis

chronic progressive form of pneumonia with wheezing, dyspnea, and diffuse infiltrates seen on radiographs; occurs following exposure to any of a variety of antigens, sometimes occupational, and many names are given to cases with known types of exposure (such as farmer's lung, maple bark stripper's lung, chicken plucker's lung, bagassosis, byssinosis, and humidifier lung); biopsy findings usually show patchy infiltration of alveolar walls with lymphocytes, plasma cells, and other inflammatory cells; can progress to irreversible interstitial fibrotic disease with restrictive pattern on pulmonary function, but in early disease most manifestations are reversible if offending antigen is identified and removed from environment.

hypersensitivity pneumonitis

an inflammatory form of interstitial pneumonia that results from an immunological reaction in a hypersensitive person. The reaction may be provoked by a variety of inhaled organic dusts, often those containing fungal spores. The disease can be prevented by avoiding contact with the causative agents. Classification of the disease is based solely on the character of the immune response rather than on its clinical manifestations. A wide variety of symptoms may occur, including asthma, fever, chills, malaise, and muscle aches, which usually develop 4 to 6 hours after exposure. Laboratory examination of the blood commonly reveals leukocytosis. Recovery is usually spontaneous. In an acute attack, corticosteroids may be given to diminish the inflammatory response. Kinds of hypersensitivity pneumonitis include bagassosis, cork worker's lung, farmer's lung, humidifier lung, and mushroom worker's lung. Also called extrinsic allergic alveolitis. See also Arthus reaction.

hypersensitivity pneumonitis

Inflammation of the alveoli due to hypersensitivity to inhaled organic dusts, fungi and moulds, which, if chronic, can lead to interstitial lung changes on chest films.

Clinical findings
Cough, fever, shortness of breath, wheezing.

hypersensitivity pneumonitis

A disorder caused by an exuberant pulmonary reaction to aerosolized immunogens, which with time may develop interstitial lung disease; the prototypic HP is farmer's lung Clnical Fever, malaise, cough, chest tightness, myalgias. See Faeni rectivirgula, Farmer's lung, Humidifier lung.
Hypersensitivity pneumonitis
Syndrome  Suspected antigen
Bagassosis Thermoactinomyces vulgaris
Bird fancier's lung Bird droppings
Pigeon handler's lung Histoplasma capsulatum
Cheese worker's lung Penicillium spp
Detergent worker's lung  Bacillus subtilis enzyme
Fish meal lung Fish proteins
Farmer's lung  Micropolyspora faeni, Thermoactinomyces vulgaris
Humidifier lung  Thermophilic actinomycetes
Malt worker's lung Aspergillus clavatus
Maple bark disease Cryptostroma corticale
Mushroom picker's lung Micropolyspora faeni
Sequoiosis Graphium spp, Auerobasidium spp
Suberosis (oak bark) Micropolyspora faeni
Vineyard sprayer's lung Copper
Wood dust pneumonitis Oak and mahogany dust
Wood pulp worker's lung Alternaria spp

ex·trin·sic al·ler·gic al·ve·o·lit·is

(eks-trin'zik ă-lĕr'jik al-vē'ŏ-lī'tis)
Pneumoconiosis resulting from hypersensitivity to organic dust, usually specified according to occupational exposure; in the acute form, respiratory symptoms and fever start several hours after exposure to the dust; in the chronic form, there is eventual diffuse pulmonary fibrosis after exposure over several years.
Synonym(s): hypersensitivity pneumonitis.

hypersensitivity

1. a state of altered reactivity in which the body reacts with an exaggerated immune response to a foreign agent; allergy is a synonym for hypersensitivity. anaphylaxis is a form of hypersensitivity.
There are four basic types of hypersensitivity reactions: Type I (called also immediate hypersensitivity) involves cell-fixed antibody, mainly IgE attached to mast cells or basophils. Antigen binding causes the cell to release vasoactive factors. The basis for anaphylaxis and atopy. Type II causes cell destruction (cytotoxicity) by the action of immunoglobulin with complement or cytotoxic cells. Seen in red blood cell transfusion reactions and in alloimmune hemolytic anemia. See also antibody-dependent cellular cytotoxicity. Type III (called also immune-complex or subacute hypersensitivity) causes tissue damage and inflammation by the deposition of antigen-antibody complexes that activate complement and attract polymorphonuclear cells. Type IV (called also delayed hypersensitivity) involves sensitized T lymphocytes that react with cell bound or associated antigen and release lymphokines, causing mononuclear cell accumulation, tissue damage and inflammation, typically manifesting at least 24 hours after exposure to the antigen.
2. a state of increased responsivity to physical stimuli.

hypersensitivity angiitis
variant of polyarteritis nodosa; a disease of small blood vessels in humans; called also leukocytoclastic vasculitis.
antibody-mediated hypersensitivity
types I, II and III hypersensitivity reactions. Called also immediate hypersensitivity.
bacterial hypersensitivity
immune responses to bacteria or bacterial products may contribute to the clinical features of some diseases, e.g. the anemia associated with salmonellosis, arthritis in erysipelas of pigs, intestinal lesions in Johne's disease, or be the principal cause as in staphylococcal hypersensitivity dermatitis in dogs.
contact hypersensitivity
a type IV reaction produced by contact of the skin with a low-molecular-weight chemical substance having the properties of a hapten in a sensitized individual; it includes allergic contact dermatitis.
cutaneous basophil hypersensitivity
a delayed inflammatory response characterized by large numbers of basophils.
cytotoxic hypersensitivity
type II hypersensitivity.
delayed hypersensitivity
type IV reaction. A slowly developing cell-mediated immune response in which T helper 1 lymphocytes respond to specific antigen by releasing cytokines, some of which activate macrophages, as occurs in tuberculin reaction, graft rejection, some autoimmune diseases, etc.
drug hypersensitivity
may be either an immediate (antibody mediated) or delayed type (T lymphocyte mediated) reaction. See also drug eruption.
flea bite hypersensitivity
see flea allergy dermatitis.
food hypersensitivity
hypersensitivity reaction to various dietary constituents has been the suspected cause of allergic dermatitis in most species, but conclusive evidence is often lacking. It may also result in diarrhea.
fungal hypersensitivity
may contribute to the clinical features of cutaneous fungal infections, particularly kerion formation. It is also the basis for skin testing for systemic mycoses, e.g. histoplasmin and coccidioidin.
helminth hypersensitivity
occurs, e.g. the self-cure phenomenon, and the allergic response of a sensitized animal to an invasion, e.g. of lungs, causes massive pulmonary edema.
immediate hypersensitivity
antibody-mediated hypersensitivity, i.e. types I, II and III, characterized by a response that appears within minutes to hours, resulting either from a release of histamine and other mediators of hypersensitivity from IgE-sensitized mast cells, causing increased vascular permeability, edema and smooth muscle contraction (type I), from antibody-mediated lysis of red blood cells (type II), or from immune complex mediated pathology (type III).
immune complex hypersensitivity
type III hypersensitivity (above).
mold hypersensitivity
see acute bovine pulmonary emphysema-edema.
hypersensitivity pneumonitis
see hypersensitivity pneumonitis.
staphylococcal hypersensitivity
see bacterial hypersensitivity (above).
hypersensitivity threshold
a theory that certain levels of allergens may be tolerated by some sensitized individuals without manifestations of disease, but a slight increase in the level precipitates clinical signs.
tuberculin type hypersensitivity
the classical T lymphocyte cell-mediated hypersensitivity associated with mycobacterium infection or immunization with antigens containing Freund's adjuvant.

pneumonitis

inflammation of lung tissue. See also pneumonia.

feline pneumonitis
see feline pneumonitis.
hypersensitivity pneumonitis
a local type III hypersensitivity reaction resulting from inhalation of antigens, seen in cattle fed moldy hay and the group of diseases in humans that includes farmer's lung and bird-fancier's lung.