Family Doctor Books

Published in association with the British Medical Association

Family Doctor Books

Preview of Understanding Allergy

What is allergy?

Humans have a sophisticated defence system that has evolved to protect us against a range of threats, including micro-organisms (such as bacteria, viruses and parasites), chemical substances and even cancer. This defence system, the immune system, is made up of a large number of different cell types and special proteins that work together in a complex way to enable us to distinguish our own cells (‘self’) from harmful cells (‘non-self’) and so to destroy abnormal or invading cells. However, occasionally the immune system reacts to sub-stances that are harmless, and the resulting allergic reaction causes damage to the surrounding tissues.

An allergic reaction is an overreaction of the immune defence system, in which it responds inappropriately to a normally harmless environmental substance, causing troublesome or even life-threatening effects. The term ‘allergy’ is sometimes used to describe just about any kind of illness. In this book, the term will be used much more precisely to mean a heightened or exaggerated response of the immune system to a substance that is normally harmless.

The different parts of the immune system include the white blood cells, the spleen, the lymph nodes, the thymus gland, and the many small glands within the lining of the respiratory and intestinal tracts. There are several different types of white blood cell, including lymphocytes, neutrophils, eosinophils, mast cells and macrophages. All of these are controlled by protein messengers (hormones), which are produced by the white blood cells.

The prime purpose of the immune system is to protect us from micro-organisms that might otherwise kill us. When one of these, such as the measles virus or the bacterium Staphylococcus, attacks the body for the first time, cells within the lymph nodes, lungs or bowel recognise it as foreign (non-self) because of certain protein molecules (known as antigens) on its surface, and bring it to the attention of the lymphocytes. One type of lymphocyte (T lymphocyte) produces protein messengers that stimulate other cells (the B lymphocytes) to produce purpose-built proteins called antibodies. These antibodies are specifically made to fit with and bind on to the antigens on the invading cell’s surface. Once there, they send signals to teams of killer cells, which move in and kill the invader. Each time a different antigen is met, antibodies are made specifically against it, and the body can make millions of different ones.

The process of recognising an antigen and making antibodies against it for the first time is called sensitisation. It may take a few days for the body’s response to reach full strength. However, for the rest of your life your immune system is able to remember harmful micro-organisms and, if the same organism attacks again, it is immediately recognised. No other organ of the body (apart from the brain) has the property of memory.

If the same micro-organism strikes again, as soon as its antigens are recognised, the T lymphocytes send chemical messages to the group of B lymphocytes that have a specific memory for the invader, and these multiply very quickly and produce large amounts of antibody, which helps to kill the harmful cells. Other white cells produce chemicals such as histamine and leukotrienes, which increase the blood supply to the area involved and make the blood vessels more leaky. This allows other white blood cell types, such as the macrophage, which are able to eat and destroy the invading cells, to reach the area. We can see evidence of this process when a skin wound becomes infected. The area becomes red and swollen because of the increased blood supply, and may be hot and sore as a result of certain chemicals produced in the course of the immune reaction. This process is known as inflammation.

How the immune system deals with harmful cells

The immune (defence) system is made up of many different cell types that work together to enable the body to distinguish between its own cells (‘self’) and harmful cells (‘non-self’). The following sequence of five illustrations is a representation of how the immune system works to destroy invading micro-organisms.

If you develop an allergy it is because, even though your immune system works perfectly well against the antigens associated with viruses, bacteria and parasites, it also reacts to other antigens that should be completely harmless. These antigens are known as allergens. These are incorrectly seen as being dangerous by your immune system, which mounts an immune response against them. This response is called an allergic reaction, and you will have developed an allergy.

There are two stages in the development of an allergy, and the first is called sensitisation. This process happens when an allergen is encountered by the immune system and antibodies are made against it, despite it being harmless. These antibodies are of the same type as those that protect us against parasites such as worms, flukes and amoebae, and are known as immunoglobulin E or IgE for short – immunoglobulin is another name for an antibody. Parasites are much larger than other micro-organisms such as viruses and bacteria, and the body has had to find alternative ways of getting rid of them. IgE is different from other types of antibody in that it can attach itself to mast cells and basophils. These white blood cells contain thousands of toxic granules capable of killing parasites, which are released when an allergen binds to IgE on the cell surface.

After sensitisation, the immune system retains a memory of the allergen and will recognise it if it meets it again. Sensitisation does not produce any symptoms and you will not be aware that it is happening. You do not always become sensitised the first time that your body meets a particular allergen – a substance may be tolerated for many years before an allergy develops.

Once you have been sensitised, even a tiny quantity of that allergen can lead to an allergic reaction. The allergen binds to the IgE present on the surface of the mast cells and basophils, and the toxic granules are released. These contain potent irritant chemicals such as histamine and a number of different enzymes. If this reaction were the result of a parasitic infection, these chemicals would help to kill and digest the invading organisms. If, however, the immune system is reacting against a harmless allergen such as pollen, these substances serve no useful purpose, but instead cause an increase in the blood supply to the tissues, leakage of fluid from the small blood vessels and local irritation. The result is hotness, redness, itching and swelling in the affected area, and the production of excess watery secretions. In addition, the muscles of the airways in the lungs and the bowel may go into spasm, causing wheezing, shortness of breath, abdominal colic and diarrhoea. This process gives us the symptoms that we associate with allergy.

Once you have developed an allergy, your body will always produce an allergic reaction whenever it meets this allergen again, even in minute amounts. However, the reaction will not necessarily be exactly the same each time. A number of things can affect the type and the extent of an allergic reaction. These include the amount of allergen involved, the part of your body exposed to the allergen, whether there are other factors present that enhance your allergic reaction (for example, a high level of air pollutants), and even how well you are at the time. Allergies can also become stronger or weaker as you get older.

What are the most common allergens?

Almost all allergens are made from proteins – organic compounds made of hydrogen, oxygen and nitrogen that are found in all living organisms. Occasionally, a non-protein substance can act as an allergen (for example, penicillin and other drugs), but only because they can attach themselves to a small protein molecule called a hapten inside the body. Virtually any protein can act as an allergen and new ones are being discovered all the time. The allergens most commonly causing allergies in the UK are described below.

House-dust mites

The house-dust mite is too small to be seen by the naked eye. It loves warm, humid conditions and large numbers can be found inside mattresses, bedding, pillows, soft furnishing and cuddly toys. House-dust mite allergy has become more common in recent times because double glazing, loft and wall insulation, and other energy-saving measures reduce ventilation inside our houses and encourage the warm, moist conditions that the house-dust mite needs.

The mite lives off scales of human skin which we constantly shed and which make up the majority of house dust. It is not the mites themselves but their faecal particles that act as allergens and cause allergies. These are so small that they can be suspended in the air for long periods of time and so can be inhaled though the nose and into the lungs, where they can cause the symptoms of hay fever and asthma. When they come into direct contact with the skin they can cause eczema. At least three-quarters of all allergy suffers in the UK are allergic to the house-dust mite.

Grass and tree pollen

Pollens are the male reproductive part of plants (or trees) that fertilise other plants of the same species. They are small grains and are carried either by insects or by air currents. The airborne pollens are very light and can be carried in the air for long periods of time. When these reach the eyes, nose or lungs of a sensitised person, they cause an allergic reaction commonly described as hay fever. If breathed into the lungs they can make asthma worse.

Different plants produce their pollens at different times of the year and it is possible, using a pollen calendar, to work out which plants might be responsible for each individual’s symptoms. The most common culprit is grass pollen, which is around from late April to early September.

Animal dander (skin scales and hair)

Most furry animals can cause an allergy. Allergens are in the skin scales, hairs and sometimes the saliva of animals. They can be spread widely throughout our homes, not just by animals but also on our clothing and shoes.

Seven of ten British households have a pet of some kind and pet allergens may cause about 40 per cent of all childhood asthma. Allergies to pets not only occur in their owners. The allergens are so potent and long lasting that they are easily spread – even non-pet owners can be exposed to enough allergen in their everyday life to become allergic.

Food allergens

Many foods can act as allergens. The most common in children under the age of five are cows’ milk and hen’s eggs. Later in life, other allergies are more common – such as to peanut (which is not a true nut but a legume, that is, it grows as a seed in a pod like a pea), tree nuts, fish and shellfish.

Food allergies can take different forms, ranging from episodes of mild symptoms such as tingling and swelling of the lips to life-threatening attacks.

Moulds and spores

The spores from certain moulds and plants can trigger allergic reactions in some people, particularly in the autumn.

Insect stings

Although everyone finds insect stings unpleasant, in some people these can trigger allergic reactions that can be severe. Stings by bees and wasps are most commonly responsible.

Drug allergies

Certain drugs, particularly some antibiotics, can cause allergic reactions. An allergy will never occur the first time a drug is administered but may do so on any subsequent occasion, even if the drug has been well tolerated in the past.

Who develops allergies?

Although we do not completely understand why it is that some people develop allergies and others do not, it is clear that allergies have a tendency to run in families. This inherited tendency towards allergy is called atopy.

Atopy is an inherited predisposition towards developing an allergic disorder. In the near future it is likely that the genes (a gene is a small part of our genetic code made of DNA) responsible for atopy will be identified.

Atopic people have the ability to produce excess quantities of the allergy antibody (IgE) when they come into contact with substances in their environment that can act as allergens.

Although atopy is inherited, environmental factors also play a part in the development of allergic disorders. This is why not all members of a family, indeed not even both of a pair of identical twins, are affected to the same extent. Factors operating in early life, probably even during pregnancy, act together with the ‘dose’ of allergy genes that you inherited from your parents to determine whether or not allergy develops.

These early life factors include the timing of your first exposure to the allergen and the size of that exposure – however atopic you are, if you were never exposed to any allergen, you would not develop allergies. The number of viral infections suffered in early childhood may also have an effect – these infections seem to be protective against allergy. High levels of exposure to tobacco smoke in pregnancy and early life increase an individual’s risk of becoming atopic.

So, a baby born to cigarette smoking, cat-owning, atopic parents, whose birth coincides with the pollen season, who spends his or her first months in a well-insulated double-glazed house, and whose early life diet contains high levels of allergenic foods has a greatly increased risk of developing allergies.

Why is allergy becoming more common?

The single most important risk factor for developing allergic disease is being born to a mother or father who him- or herself has allergic disease. However, there has been a marked rise in the number of people suffering from allergies within just one generation and, as we know that changes in the genetic make-up of our population take many generations to occur, other factors must be involved. These include the following.

Infant feeding

If babies are exposed to food allergens such as cows’ milk and eggs early in life they are more likely to develop allergic disease. Breast-feeding does not entirely protect the infant because allergens from food eaten by the mother can be secreted in breast milk.

Although current Department of Health guidelines suggest that solid food should not be introduced before the age of four months, many babies are started on solids rather earlier. The Western diet contains increasing amounts of commercially prepared food containing preservatives and other chemicals, and decreasing quantities of foodstuff such as fresh fruit and vegetables that contain protective antioxidants. Antioxidants are substances that slow down the breakdown of another substance by oxidation. They can also mop up free radicals, molecules that could otherwise damage healthy DNA. The most important antioxidants are vitamins A, C and E and selenium, with high levels being found in fresh fruit and vegetables.

Allergies have a tendency to run in families. This is called atopy. Environmental factors also play a part in the development of allergic disorders. This is why not all members of a family are affected to the same extent.

Delaying the introduction of solid food appears to protect against allergies, especially eczema. The incidence of eczema by age two is directly related to the number of different solid foods taken by the baby before the age of four months. If a breast-feeding mother herself avoids highly allergenic food such as milk, eggs, peanuts and fish, an extra protective effect is seen.

Other allergens

Exposure to high levels of air-borne allergens in very early life appears to increase an infant’s risk of developing allergic disease. Babies born in spring when the pollen count is high are more likely to be allergic to plant pollens at age ten than children born at other times of the year. Similarly, babies exposed to pet allergens in very early life have an increased risk of allergy. Although sensitisation may occur at any age, the first year of life appears to be particularly important, and there is increasing evidence to show that exposures in mid and late pregnancy are also important.

Tobacco smoke

Exposure to tobacco smoke both before (via the mother’s bloodstream) and after birth is strongly associated with the development of allergies and allergic disease, particularly asthma. Babies born to smoking mothers have raised levels of IgE in their blood at birth. Exposure to smoke after birth increases the risk – children living in smoking households are twice as likely to be admitted to hospital with chest illness as children living in non-smoking households. These children also have significantly reduced lung function by age seven. Passive tobacco smoke exposure is the strongest identified risk factor for the development of allergic disease and so it is particularly alarming to see that smoking is still rising in popularity in young women of childbearing age.

Indoor environment

European children spend at least 90 per cent of their time indoors so the indoor environment is probably more important than all other geographical and outdoor environmental factors. Modern buildings tend to be well insulated and have poor levels of ventilation, and these factors appear to be a risk factor for the development of allergies. This may be the result of increased levels of chemical fumes from materials such as plastics and synthetic paints, high humidity leading to indoor mould growth and increased levels of house-dust mite allergen. These living conditions seem to exert a particularly powerful effect in atopic children – already at risk because of a family history of asthma.

Infections and antibiotics

Clear evidence is emerging that frequent viral and bacterial infections in early life may protect against the development of atopy and allergic diseases. Early life infections encourage the production of a chemical called interferon-gamma, which is found in higher levels in non-allergic people than in allergic people. If our children develop lower levels of interferon-gamma because we protect them from catching minor infections when they are young, we may be inadvertently increasing their chances of developing allergic disease.

The overuse of antibiotics may have a similar effect. In contrast, children who have large numbers of older brothers and sisters, and who go to child-care centres where they mix with large numbers of other children (and their germs), are less likely to develop hay fever and asthma.

However, it is vital that we continue to protect our children from dangerous infections such as polio, tetanus, whooping cough, measles, mumps and rubella by the use of immunisations. Any benefit that might be given by not immunising is vastly outweighed by the harm done in leaving children vulnerable to catching these childhood infections; all of them are unpleasant and some can kill.

Finally

Our Western diet, with the use of formula milk and the early introduction of solid foods, in combination with our Western lifestyle (early life exposure to tobacco smoke, high levels of plant and animal allergens, house-dust mite, living in poorly ventilated humid houses, having smaller families and delayed use of child-care centres) all conspire to increase the number of children in our population developing allergic problems.

KEY POINTS

An allergic reaction is an inappropriate and harmful response of your body’s defence mechanisms to substances that are normally harmless
Sensitisation does not always occur the first time that your body meets a particular allergen, and a substance may be tolerated for many years before an allergy develops
Once you have become sensitised, even a tiny amount of allergen can produce an allergic response
Although atopy (genetic tendency towards allergy) is inherited, environmental factors also play a part in the development of allergic disorders

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