The respiratory tract is protected by a number of defence mechanisms, including:

• particle removal in the nostril/nasopharynx;

• upward removal of substances via mucociliary transport;

• presence in the lungs of immune cells, such as alveolar macrophages;

• production/presence of soluble protective factors, including a1-antitrypsin, lysozyme, lactofer-rin and interferon.

Failure/ineffective functioning of one or more of these mechanisms can impair normal respiratory function. Emphysema, for example, is a condition in which the alveoli of the lungs are damaged. This compromises the lung's capacity to exchange gases, and breathlessness often results. This condition is often promoted by smoking, respiratory infections or a deficiency in the production of serum a1-antitrypsin.

a1-Antitrypsin is a 394 amino acid, 52 kDa serum glycoprotein. It is synthesized in the liver and secreted into the blood, where it is normally present at concentrations of 2-4 g l_1. It constitutes in excess of 90 per cent of the a1-globulin fraction of blood.

The a1-antitrypsin gene is located on chromosome 14. A number of a1-antitrypsin gene variants have been described. Their gene products can be distinguished by their differential mobility upon gel electrophoresis. The normal form is termed M, but point mutations in the gene have generated two major additional forms, i.e. S and Z. These mutations result in a greatly reduced level of synthesis and secretion into the blood of the mature a1-antitrypsin. In particular, persons inheriting two copies of the Z gene display greatly reduced levels of serum a1-antitrypsin activity. This is often associated with the development of emphysema (particularly in smokers). The condition may be treated by the administration of purified a1-antitrypsin. This protein constitutes the major serine protease inhibitor present in blood. It is a potent inhibitor of the protease elastase, which serves to protect the lung from proteolytic damage by inhibiting neutrophil elastase. The product is administered on an ongoing basis to sufferers, who receive up to 200 g of the inhibitor each year. It is normally prepared by limited fractionation of whole human blood, although the large quantities required by patients heightens the risk of accidental transmission of blood-borne pathogens. The arantitrypsin gene has been expressed in a number of recombinant systems, including in the milk of transgenic sheep. Although use of the recombinant product would all but preclude blood pathogen transmission, it appears significantly more costly to produce.

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