What does the word allergy mean?
Allergy is a word which we usually use for a particular group of unpleasant or dangerous symptoms which a few people get from substances which are harmless to most of us. In fact they were almost always harmless in the past to the same person who is allergic to them now. We don't start out with allergies, but become allergic as a result of contact with the things which cause it. Just occasionally this even happens in the womb, before we are born. Just as you don't have immunity to measles unless you have been infected or immunised with measles virus, so you can't have allergy to grass pollen or nuts unless you have come into contact with grass pollen or nuts. Allergy is immunity gone wrong.
We need immunity to protect us from infections, and without our immune systems we would soon die from infection. A very important group of infections throughout human history has been infection with parasites such as parasitic worms. Even today, when worm infections have almost been banished from developed countries, worm parasites cause vast numbers of deaths in poorer countries. But even when we have got rid of the worms from our surroundings we still have the same genes which protected us from this scourge. When we get allergies like hay fever or allergy to nuts, what happens is that this unused part of our immune systems is causing mischief.

There are other ways in which our immune systems can cause allergic diseases. For example if you are allergic to nickel in jewellery or clothing, this is caused by quite a different part of our immune system. But it is still 'immunity gone wrong'.

The word "allergy" means "altered working". It was coined at the beginning of the 20th century to describe the fact that dogs immunised with venom proteins from another animal had fatal reactions when they had another injection of that protein. Instead of being protected by immunisation; the dogs died during the "allergic reaction". In fact they had a kind of reaction for which the experimenters coined the word "anaphylaxis", meaning the opposite of "prophylaxis" or protection. Vaccination against bacteria and viruses produced protection, or "prophylaxis", but the experiments showed that immunisation could also produce increased harm on further contact, or "anaphylaxis".

Since then the word "allergy" has been used in two senses. Most people use it to mean an illness produced by a reaction of our immune system to some protein or other substance.

Examples are hayfever, allergic asthma, food allergy, drug allergy, and infantile eczema.

Scientists use the word "allergy" to mean any kind of altered state of the immune system in which it reacts differently to a substance as a result of previous contact. To refer to illnesses brought on in this way, they use the expression "allergic disease".

We will use the word "allergy" to mean "allergic disease", the way most people use it.

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What kinds of allergy are there?
There are two broad types of allergy as far as most people are concerned.
The first kind is the common kind of allergy which causes hayfever, allergic asthma, food allergy, and some drug allergy. This type involves a very quick reaction, typically taking 15 minutes to become really obvious, and is called immediate hypersentitivity

The second is a peeling eczema-like rash called contact dermatitis, which some people get from metals such as nickel in jewellery, watches or clothing items. It can also be caused by cosmetics, sticking plaster, or a variety of other things which come into contact with the skin. This type of reaction is much slower, typically taking two days to become really obvious, and is called delayed hypersentitivity

These two groups of allergic illnesses are completely separate, and having one kind does not mean you are more likely to have the other kind.

We are only going to be dealing with allergies related to the first group, immediate hypersentitivity.

Illnesses commonly caused by immediate hypersensitivity are:

• Hayfever
• Infantile eczema and similar eczema later in life
• Asthma of the allergic type; most asthma, in fact.
• Food allergies

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Is allergy inherited?
Someone who gets one of these allergic illnesses is more likely to get one of the others, and as a group these problems run in families. We now know that there are a number of different genes which, when inherited, cause a person to have the tendency to get these conditions.
This tendency is called atopy. If you have the tendency you are atopic. Atopy is the tendency to develop immediate hypersensitivity more readily than most people if substances capable of causing this kind of allergy come into contact with the surface membranes (so-called mucous membranes) of your digestive system or your respiratory system. Possibly this also applies to some things which come into contact with the skin.

It is obviously possible to inherit more than one such gene, and some atopic people are more atopic than others. If you are more atopic you are likely to become allergic to a lot of things. If you are only slightly atopic you are likely to become allergic to only a few.

It makes no sense whatever to avoid having children because you have allergy genes. A third of the human race has these genes, so there will always be untold numbers of people with these genes about, whatever you do. What is more, we know that even if you have the genes, it still depends on our environment whether allergies develop. People with these same genes did not have so many troublesome allergies in the past, and we expect that in the future we'll know why and will be able to stop allergies from happening. If you have a terrible form of allergy, it is very unlikely that your children will have such bad allergies; so much depends on other things. So many other things are more important than whether you have allergy that your decision to have children should not be influenced at all by allergy.

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What about the environment?
Whether you actually get allergies depends not only on your genes, but also on your environment. If you live in a country without pollen you will not get hayfever. But other aspects of our environment must be involved too.
Air pollution has been blamed for increased hayfever in Japan, and may have played a part in causing hayfever in other countries.

The month in which you are born makes a slight difference to your chance of getting hayfever. It seems that if you are exposed to things which cause allergy in the first three to six months of life, you are more likely to be allergic to them later in life. In other words, there is a 'vulnerable period' in early childhood. This has given rise to the idea of allergy prevention by avoiding early contact with things which cause allergy.

Surprisingly, your chance of having infantile eczema, hayfever, or positive allergy skin test results goes down if you have more older brothers and sisters.Why? Well, it seems that people with more brothers and sisters when they are little get more infections at that age, and make lots of antibody to bacteria and viruses. When our immune system gears itself up to make lots of the common type of antibody against germs, it can't make lots of allergy antibodies at the same time.
Scientists are looking at the possibility of boosting the kind of antibody response we make against germs in order to protect children against allergies later in life.

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Is allergy increasing?
All over the world there is evidence that allergic diseases are increasing. Allergy seems to be a problem of 'Westernised' societies. The figure shows increases in Aberdeen, UK, schoolchildren from 1964 to 1989.

The increase in asthma, hayfever and eczema in Aberdeen children; questionnaire surveys 1964 and 1989.
SOB = shortness of breath.

Why should allergies be increasing? One piece of evidence comes from a report that a person's chance of having hayfever is higher if he or she has few or no brothers or sisters, and lower if more brothers or sisters. D. Strachan, who reported this in 1989, thought an explanation for this might be that children with more brothers or sisters have more infections in childhood, and that the infections might protect against allergy. Subsequent immunological research has provided an explanation for this.

Air pollution from road traffic has been blamed. The main evidence for this has come from Japan, where 'hayfever' due to the pollen of the Japanese cedar tree has increased dramatically since the second World War. The increase seemed prominent particularly where there was diesel traffic rather than where the cedar trees grew. Experiments showed that mice produced the allergy-causing antibodies much more readily if particles from diesel exhaust fumes were put into the nose. However, attempts to confirm the effect of traffic fumes on nasal allergy in Europe have run into difficulties. The evidence for a link between asthma and air pollution is weaker still, though a lot of research is going on and some of this has provided evidence that air pollutants can, in the laboratory, produce changes which would be expected to worsen asthma.

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