Allergies come in many forms and can appear and disappear at different stages of life.
They’re especially common in children, although adults can develop allergies to things that have never previously been a problem. Over a quarter of people in the UK are thought to be affected by allergies at some point in their lives.
Allergies often arrive in the form of airborne particles - pollen, dust and dander being prime culprits. These allergies understandably tend to affect the parts of the body concerned with the respiratory system.
Other allergies can come about through insect stings, or reactions to food or medication. Here, reactions can include diarrhoea, vomiting, stomach pain and skin irritation. Allergic reactions can also be caused by materials such as latex coming into contact with the skin. Skin allergies can manifest in the form of reddening or swelling.
In severe reactions to allergens, the body may suffer an allergic response called anaphylaxis (or ‘anaphylactic shock’). This can cause symptoms including dramatic swelling, low blood pressure and, if not properly and promptly treated, can lead to coma or death.
Allergic reactions are triggered, ironically, by the body trying to protect itself from perceived threats. Immunoglobulin antibodies, part of the body’s immune system, attach themselves on one end to an allergen and on the other, to a particular type of white blood cell, causing the cell to degranulate. This means the cell releases its store of compounds such as histamine, which then cause inflammation.
So why are some people more prone to allergies than others? There are a number of so-called ‘host’ factors, of which the most significant is heredity. Curiously, if you have parents with allergies you’re statistically more likely to develop them yourself - though not necessarily the same ones. Environmental reasons are also believed to play a part, including pollution and changes to one’s diet.
Another popular theory is that we are now more prone to allergies as we’re ‘too clean’. The so-called ‘hygiene hypothesis’ suggests that not being exposed to enough infections and parasites in childhood leads to an underdeveloped immune system and so to more illnesses and allergies in later life.
This may in part explain why allergies can suddenly develop in adulthood, but there are other possible causes. One is being exposed to allergens when the immune system is weakened, such as during illness or pregnancy. It’s possible that an adult-onset allergy is actually the recurrence of a forgotten childhood problem. Moving to a new area with a different environment - one with higher pollution or different flora - can also trigger dormant allergies. And in many cases, it’s simply that the threshold of exposure to whatever causes the allergic reaction is sufficiently high for it only to be reached in adulthood.