What is allergy?

The body’s defence system

The human body has 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. These work together in a complex way to enable us to distinguish our own normal cells (‘self’) from harmful cells (‘non-self’) and so to destroy abnormal or invading cells. However, occasionally the immune system reacts to substances that are harmless.

An allergic reaction is an overreaction of the immune defence system to a normally harmless environmental substance. The resulting allergic reaction causes damage to the surrounding tissues, leading to troublesome or even life-threatening consequences.

The term ‘allergy’ is sometimes used to describe just about any kind of illness. In this book, the term is used much more precisely to mean an exaggerated and inappropriate response of the immune system to a substance that is normally harmless.

An allergic reaction involves different parts of the immune system, including 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 cells, including lymphocytes, neutrophils, eosinophils, mast cells and macrophages. All of these are controlled by hormones (chemical messengers that trigger specific effects in the body).

In the healthy person

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 aureus, attacks the body for the first time, cells called macrophages 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 these cells bring it to the attention of the lymphocytes (white blood cells).

One type of lymphocyte, the 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 onto 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 each harmful micro-organism 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 unique antigens are recognised, the T lymphocytes send chemical messages to the group of B lymphocytes that have a specific memory for that invader.

These multiply very quickly and produce large quantities of antibody, which helps to kill the harmful organisms. Other white cells produce chemicals such as histamine, which increases the blood supply to the area involved and makes the blood vessels more leaky. This allows other helpful 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 the effects of histamine 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 the chemicals produced in the course of the immune reaction. This process is known as inflammation.

In the allergic person

When you develop an allergy, your immune system still works perfectly well against the antigens associated with viruses, bacteria and parasites, but, in addition, 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.

Sensitisation

There are two stages in the development of an allergy, and the first is sensitisation. This process happens when an allergen is encountered by the immune system and antibodies are made against it, despite it being harmless.

These are a special type of antibody, known as immunoglobulin E or IgE for short – immunoglobulin is another name for an antibody. They are of the same type as those that protect us against parasites such as worms, flukes and amoebae.

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 special cells called mast cells and basophils, which contain thousands of toxic granules. These toxic granules are released when an allergen binds to the IgE on the cell surface, causing the symptoms that we associate with allergy.

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. Sensitisation does not produce any symptoms and you will not be aware that it is happening. After sensitisation, the immune system retains a memory of the allergen and will recognise it if it meets it again.

Allergic reactions

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 even minute amounts of this allergen again. 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 or dose 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 unwell you are at the time. In addition, the pattern of your allergic reactions may change as you get older. Some may become less pronounced with age, and it is even possible to ‘grow out of’ an allergy.

What are the most common allergens?

All allergens are made of proteins – organic compounds made of hydrogen, carbon, 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 furnishings 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 an allergen and cause allergies. These particles 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 sufferers 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 consist of small grains and are carried to another flower by either insects or the wind. Wind pollination is not very efficient, and so plants that reproduce in this way produce vast quantities of pollen. The airborne pollens are very light and can be carried in the air for long periods of time. When these particles 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, in addition, 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 your 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 as a result of the allergens in their skin scales, hair, urine and saliva. These allergens can be spread widely throughout our homes, not just by animals but also on our clothing and shoes.

Seven out of ten British households have a pet of some kind and pet allergens cause about 40 per cent of all childhood asthma. Allergy to pets does not only occur in their owners. These allergens are so potent and long lasting that they are easily spread, for example by contact with bus seats, to non-pet owners, who can be exposed to enough allergen in their everyday life to become sensitised.

Food allergens

Many foods can act as allergens. The most common in children under the age of five are cows’ milk and hens’ eggs. Over the age of five years, other allergies are more common – such as to peanuts (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 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.

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.

Insect stings

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

Who develops allergies?

Family history

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.

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

Environmental factors

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 degree 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 the development of 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 has allergic disease him- or herself. 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 cannot occur that quickly, 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 six months, many babies are started on solids rather earlier. Our western diet contains increasing amounts of commercially prepared food containing preservatives and other chemicals, and decreasing quantities of certain foodstuffs 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 (genetic material). The most important antioxidants are vitamins A, C and E and selenium, with high levels being found in fresh fruit and vegetables.

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, as all of these substances can pass into breast milk.

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 relevant, 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 with a mother who smokes are four times as likely to develop wheezing and 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 most popular 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, who are 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 produce 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 a large number of older brothers and sisters, and who go to childcare centres where they mix with large numbers of other children (and their germs), are less likely to develop hay fever and asthma.

Immunisations

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 theoretical benefit on the incidence of allergy that might result from 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 rather than breast 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 childcare 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, especially tobacco smoke exposure, also play a part in the development of allergic disorders