The rise and rise of asthma
Glossary
airways. A general term for the system of tubes that runs from the back of the mouth and nose into the lungs. The largest is the trachea or windpipe. In the chest, the trachea divides into two smaller tubes called bronchi. Each bronchus then supplies one lung. After entering the lung, the bronchi divide further into narrower tubes called bronchioles and these supply the air sacs of the lung. The airways contain a thin lining on the inside moistened with a little mucus. Barring choking or drowning, from the moment you are born your airways are kept clear and, unless you smoke, fairly clean. Asthma sufferers find that periodically their airways narrow and they can't breathe normally. The smooth muscle is contracting and so narrowing the airway, and at the same time the inner lining becomes inflamed and reddened. It may also swell. Extra mucus is produced, sometimes thicker than usual, and it can partially clog or obstruct the airway.
allergens. An allergen is any substance that triggers an allergic reaction. Common respiratory allergens are grass pollen, mould spores or house dust mite faeces (present in dust); other allergens may affect the skin or the digestive system.
allergic reaction. Allergies are inappropriate reactions of the immune response to substances (allergens) that normally wouldn’t cause any noticeable effects. Most allergic reactions involve the allergen binding on to special immune system cells and causing these cells to release compounds that affect the surrounding tissue. One such compound is histamine. It causes itching and inflammation. Chemicals that block the effect of histamine are called antihistamines, and they are standard allergy medication. However, they are not particularly effective in asthma.
bronchodilators. (Also called 'relievers'.) These are a group of drugs that relax the smooth muscle in the airway walls and hence widen (dilate) the airways. Used to relieve the symptoms of an asthma attack.
chronic. Used to describe a medical condition that continues for a long time, often with little change. A chronic disease, such as asthma, may have acute episodes, when the situation worsens for short periods of time.
cytokine. A hormone-like molecule, produced by one cell, that has an effect on another cell. Some types of cytokines are normally produced in low concentration by the immune system as part of the body's defence mechanism. Some cytokines are toxic at high concentrations and cause disease symptoms such as fever.
eosinophil. A white blood cell that increases in number as a result of certain parasite infections and allergic diseases.
epithelium/epithelial. A tissue composed of cells that line the cavities and surfaces of structures throughout the body. In the lung, the epithelium is a thin layer of cells which lines the airway tubes in order to protect and regulate the tissue underneath.
extrinsic. External or a cause coming from outside. In this type of asthma, the cause of an attack is normally the inhalation of an allergen. Extrinsic asthma is more likely than intrinsic to start in childhood, and often the trigger(s) can be identified and dealt with. In extrinsic asthma, the reaction of the airways is like an allergic reaction, and is similar to hayfever and other allergies.
gene. The basic unit of inheritance. A gene is a segment of DNA that specifies the structure of a protein or an RNA molecule.
house dust mites. Tiny mites (about one-third of a millimetre long) that feed off human skin flakes and bodily secretions. They colonise houses, especially in warm, humid areas. They tend to live in carpets, mattresses, pillows and soft furnishings. Although quite harmless, their droppings contain substances that are allergens. Exposure to the droppings (invisible to the eye) can cause sneezing, itchy, red eyes or asthma attacks.
inflamed/inflammation. Inflammation is the process that makes living tissue swell, become painful and turn red. Inflamed tissue contains damaged cells and has a higher than normal blood flow through it – that is why it’s red and warm. It is usually ‘infiltrated’ by many cells of the immune system. Compounds released from damaged cells cause fluid and more inflammatory cells to leak out of the blood vessels in the area; this fluid accumulates and may make the tissue swell or block tubes. Inflammation is often associated with infection but it can also be caused by allergic reactions. One of the major inflammatory cells in asthma is the eosinophil, which can damage the airway lining. This can ultimately lead to permanent damage in the airways. Inflammation of the lining of the nose, for example, causes the blocked nose characteristic of colds or of hayfever. Inflammation of the airways occurs in asthma, but is not unique to it.
interleukin. A chemical messenger secreted by cells of the immune system. They act by affecting the behaviour of the rest of the immune system. For example, they may attract immune system cells to an area of the body or they may stimulate the development of some cells of the immune system.
intrinsic. Instrinsic asthma has no clear connection with allergy. It can start at any age. The triggers are usually infection, polluted air, exercise, or cold temperatures, but some attacks occur without any obvious trigger.
nitrogen oxides. Chemical formula NOx. This covers the gases nitric oxide (chemical formula NO) and nitrogen dioxide (chemical formula NO2 ). Both can be toxic but nitrogen dioxide is considered to be of most concern for asthmatics. The main source of the gases in urban areas are motor vehicle exhaust and gas cookers and kerosene heaters indoors. The brown haze sometimes seen over cities is mainly nitrogen oxides. These gases are also partly responsible for the generation of ozone, when acted upon by sunlight in the presence of other chemicals. Although air pollution can cause irritating symptoms and increased asthma symptoms in some people, it is unlikely to be an important cause of asthma in Australia.
microRNA. A short piece of single-stranded RNA that does not encode a protein. They have a number of functions including regulating the expression of genes.
occupational sensitisers. Chemicals or compounds that causes airway inflammation leading to asthma. These sensitisers occur in particular occupations such as carpentry (eg, western red cedar wood dust is a sensitiser) and commercial spray painting (some duco paints are sensitisers).
ozone. Ozone (O3 ) is a form of oxygen. It is a colourless gas that has a very pungent odour. It exists naturally at low concentrations in the stratosphere where it absorbs ultraviolet radiation. In the troposphere it exists naturally at extremely low concentrations. But these concentrations increase when sunlight acts on various gases, coming mainly from vehicle exhausts, and ozone then becomes a pollutant in the troposphere. Ozone is a highly corrosive gas and is poisonous to most organisms. At concentrations as low as 0.00001 per cent (or 10 parts per hundred million) it can irritate the membranes lining the nose, throat and airways and can trigger or exacerbate asthma attacks.
prebiotics. Non-digestible carbohydrates that stimulate the activity and growth of beneficial bacteria in the digestive system.
probiotics. Cultures of live microorganisms that can be taken to improve the balance of natural microflora in the digestive tract.
protein. A large molecule composed of a linear sequence of amino acids. This linear sequence is a protein's primary structure. Short sequences within the protein molecule can interact to form regular folds (eg, alpha helix and beta pleated sheet) called the secondary structure. Further folding from interaction between sites in the secondary structure forms the tertiary structure of the protein.
Proteins are essential to the structure and function of cells. They account for more than 50 per cent of the dry weight of most cells, and are involved in most cell processes. Examples of proteins include enzymes, collagen in tendons and ligaments and some hormones. More information can be found at Protein structure and diversity (Molecular Biology Notebook, Rothamsted Research, UK).
RNA (ribonucleic acid). A nucleic acid similar to DNA. In most organisms, RNA serves as a ‘read-out’ of the genetic information in DNA to facilitate various aspects of cell metabolism, particularly as a message for protein synthesis. However, in some viruses, including influenza, RNA is the primary genetic material instead of DNA, and is more prone to mutation than DNA. For more information see RNA (Nobelprize.org).
smooth muscle. All airways have bronchial smooth muscles in their walls. These muscles are classed as 'smooth' muscle. This means they are not under voluntary control, like the muscles of our legs and arms, but instead respond to circulating hormones and compounds released locally by damaged or inflamed tissue. Many drugs will cause changes in smooth muscle without any effects on our voluntary muscles. Smooth muscle contraction will narrow airways and can also constrict arteries and many other tubes in the body. Smooth muscle relaxation will dilate (widen) these tubes.
sulfur dioxide. Sulfur dioxide (chemical formula SO2 ) is an acrid-smelling gas that even at low concentrations irritates the membranes of the nose and respiratory system. It is thought to exacerbate many respiratory diseases, including asthma. Sulfur dioxide is produced whenever sulfur-containing compounds are burnt. Its commonest source in Australia is power-stations burning coal containing slight sulfur impurities.
T-cell. White blood cells that are important for the body's immune response to specific antigens. Killer T-cells are like soldiers who search out and destroy invading bacteria or viruses or cancer cells.
trigger. A stimulus that causes asthma symptoms or an attack. Triggers include irritants such as fumes, cigarette smoke, allergens such as house dust mite or moulds, viral respiratory tract infections, and exercise. Not every asthmatic responds to every trigger. And not every asthmatic responds to the same trigger in the same way on each exposure. Some triggers, such as allergens, can cause worsening airway inflammation.
white blood cell. (Also known as leucocytes.) White blood cells are the immune system cells. They can be divided into many different categories on the basis of their function and appearance. Many are not found in the blood at all and those that are may have the ability to crawl out of blood vessels, squeezing between the cells of the vessel walls. While some produce antibodies, others produce cocktails of destructive chemicals, others kill virus-infected cells by punching holes in them, and a further class control the entire immune response. For more information see White blood cells (Puget Sound Blood Center, Washington).
Page updated February 2011.