Since the 1960s there has been a progressive increase in the incidence of allergic disease in Western countries. Even though the processes involved with allergic reaction are largely understood there is no clear reason for this rise. Scientists have developed a number of ideas that suggest factors that lead to an increased risk of allergies. These could be used to develop greater public awareness which could in turn reduce the frequency of this disease.
In order to try and explain the recent increase, the ‘hygiene hypothesis‘ was proposed. An epidemiologist by the name of Dr Strachan came up with this idea in 1989. He suggested a link between the rise of allergic disease and low levels of infection during childhood from unhygienic environments. This has been backed up by many recent studies and fits very well with the current knowledge of the immune System. There is a type of white blood cell called the Helper T Cells that act as ‘middlemen’ during infection. Their role is mostly to promote and regulate an immune response rather than actually fight for the body. There are two main subtypes of helper T cells:
- Type 1 helper T cell (Th1): This cell is ‘normally’ used to help with standard bacterial and viral infections.
- Type 2 helper T cell (Th2): This cell is usually reserved for parasitic type infections.
During an allergic reaction it is the Th2 response that leads to the adverse effects. These cells are acting against naturally harmless substances (e.g. peanuts) resulting in the unnecessary inflammation that occurs. Not only do Th1 and Th2 cells work differently, they also regulate each other. During a normal Th1 response, these cells will not only stimulate an immune response, but also suppress the Th2 cells from reacting. It has been found that newborn infants are born with a reduced Th1 response that gives rise to a dominant Th2 system. The hygiene hypothesis works with this idea. For a person to develop normal Th1 they need to have a certain level of exposure to germs. It is said that this development occurs during early childhood and is established by 5 years of age.
In Western societies, high value is placed on cleanliness not only indoors but also on the body. By being overly hygienic, infants are not being exposed to the germs that lead to the natural development of the immune system. In 2004, comparisons were made in Sweden amongst people from farming environments and those from non-farming environments. The purpose was to compare those from a natural ‘dirtier’ environment with those from city areas where there is a focus on clean and hygienic surroundings. The results supported the hygiene hypothesis, showing that those from farming environments had a lower risk of allergic disease.
It is well known some bacteria are in fact useful and needed by our bodies. An example of this is the bacteria living in our stomach that we need in order to digest our food. There is now increasing evidence that exposure to harmless bacteria that naturally live in the body is necessary for immune development.
To try and confirm this, scientists have done comparisons between animals with a naturally occurring gut bacteria and those that have been raised germ-free. Those with the bacteria were actually found to have better immune development including proper Th1 and Th2 development. Some bacteria known to stimulate the immune system properly are actually found more frequently in non-allergic children. The idea coming from these types of studies suggest that if children do not come across these bacteria then another type of bacteria may take its place.
The use of antibiotics at an early age can quite easily disrupt bacteria and its impact on the immune system, whether it is infectious or harmless. There is a fair amount of evidence that supports this but it is still uncertain whether other factors are involved. A study in Germany showed that six or more courses of antibiotics in the first year of life were linked with a later excess of hay fever and eczema. Using antibiotics at an early stage of life seems to have a two-pronged effect. Not only can it kill a large amount of good bacteria, but it also lowers this bacteria’s ability to stop bad bacteria growing in its place.
More research needs to be done in this area as it may simply be what is called ‘reverse causation’. Some researchers believe that increased antibiotic prescription during childhood is caused by early symptoms of asthma and hay fever. In this sense, it is the pre-existing allergy that is causing increased antibiotic use and not the other way around.
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