The latest surveys show that the rates of allergy are increasing throughout the world, affecting up to 30-35% of people at some stage in their lives. This increase was initially seen in countries such as the UK, Europe and USA, but can now be found in all countries undergoing industrial development. The pattern of allergy is also changing – initially, the increase was in asthma and allergic rhinitis (hay fever). However, recent studies have confirmed a significant increase in the incidence of food allergies, in particular amongst children. In the UK, it is estimated that up to 50% of children are diagnosed with an allergic condition.
Allergy is caused when the body's immune system reacts to a normally harmless substance, such as pollen, food, or house dust mite. The body identifies the substance as a threat and produces an inappropriate, exaggerated response to it. What we are only beginning to understand is what tips the balance in favour of allergy. Researchers have suggested that a number of factors might cause someone to become allergic:
Children born into families where allergies already exist have a higher than average chance of developing allergies themselves. In the UK today, children have a 1 in 5 predisposition of developing an allergy. However, the risk is doubled if one parent has an allergy (particularly if that parent is the mother). If both parents have allergies, the risk is increased to 60-80%. This increased tendency for individuals to develop allergies because of their genes is known as being atopic.
Atopy results in an increased risk to any allergy, not just a specific type of allergic disease. So a child to parents who suffer from hay fever is no more likely to develop hay fever than asthma or food allergy. Similarly, the severity of the allergy in a child bears no relation to the severity of the parents’ allergic disease.
However, within the human population, changes in genes take many hundreds of years to cause consequences in disease. So atopy/genetic tendency alone cannot account for the current increase in allergy seen over the past few decades. Current research is investigating the effect of the environment in which we live on genetic factors. It is now clear that many genes can be ‘turned on or off’ by environmental factors. For example, viral infections are able to modulate our immune systems by switching certain genes on or off, promoting an allergic tendency in immune cells.
This suggests that the immune system needs to come into contact with a variety of micro-organisms and bacteria while it is developing at the infant stage, in order that it responds appropriately later in life. We now live in an environment where we use cleaners containing anti-microbial agents, and food preparation is more hygienic than ever. Whilst children living in farms were previously directly exposed to animals, and their environment contained a range of microbial agents and plant derived agents, most of us now live in cities where we have minimal exposure to animals. We know that children with regular contact with farm animals have a lower incidence of allergy. Inadequate exposure to environmental micro-organisms may therefore result in the immune system of atopic children developing a tendency towards allergy.
Over the last 100 years, vaccines and antibiotics have radically changed our lives and defeated many infections that previously killed large numbers of the population. The continued use of vaccinations is, of course, essential to protect the health of our children and the nation, and eradicating virulent killer diseases is of the utmost importance. However, the cost of this may be the reduction in the pressure placed on immune system to mount a strong immune response against infectious agents. This may be one of the factors why many people’s immune systems are now reacting to allergens.
Our diets tend to include more processed foods and less fruit and vegetables. It has been suggested that the increase in food allergy might be due to more allergenic foods, such as peanut, in our diet. However, there is no evidence that this has happened, and many cultures traditionally eat high amounts of certain allergenic foods, for example, peanuts in some Asian communities.
A number of research teams are investigating whether reduced levels of nutrients – in particular vitamin D, omega-3 fatty acids (in fish) or antioxidants – might contribute to the development of allergy. While a diet low in oily fish has been associated with increased risk of childhood asthma and allergies in some studies, one study giving extra fish oils to babies did not prevent the occurrence of allergies.
Vitamin D is important for the immune system and in early lung development. Deficiency of vitamin D is increasing throughout the world mainly because of sunlight avoidance through spending more time indoors or using sun-screen to reduce risk of skin cancer (most vitamin D is produced by the action of natural sunlight on skin). However, the data linking vitamin D deficiency to allergy is conflicting. Furthermore, giving pregnant women vitamin D supplements does not appear to have a consistent effect on reducing the risk of allergies in children.
A large number of cosmetic products include ingredients derived from nuts and other potentially allergenic foods. Some researchers suggest that exposing the body to potential allergens through skin creams may cause the development of allergy in people at risk of allergy. There is only a limited evidence for this, and the situation is likely to be much more complicated than just applying the wrong sort of cream to a child’s skin.
Our environment today is very different from 50 years ago. While there is evidence that pollutants can exacerbate existing airway allergy, the question of whether pollution can cause new allergy remains controversial. One hypothesis for which there are accumulating data, is that the increase in allergy mirrors our declining exposure to bacteria and other micro-organisms in our environment. This has led to the Hygiene Hypothesis.
So an intriguing possibility is that many of the above dietary and environmental factors may increase allergy risk by regulating genes which promote an allergic-type immune system. Hopefully, our understanding of epigenetics will increase over the coming years, offering new potential strategies by which we might be able to prevent allergy.
Reference: Allergy UK November 2013