More and more people are affected by allergies, yet scientists aren’t sure why they even affect people in the first place. There are two leading theories: one sees allergies as a misfiring of a defence against parasitic worms, while the other as a kind of home alarm system.
An allergic reaction happens when our immune system responds to defend us against a particular food or substance, known as an allergen. The problem lies in that the allergen isn’t always harmful, which makes the allergic reaction both unnecessary and if the reaction is severe possibly harmful in itself.
More and more people develop allergic reactions, often people affected have a family history of allergies or have closely related conditions, such as asthma or eczema. In the United Kingdom around a quarter of the population is affected by allergies in their life, while over 50% of Americans are affected. Allergies are especially common in children; some allergies will go away when we get older, while others remain a lifelong nuisance. We can also develop allergies in adulthood to allergens that previously caused no issues.
Common allergens include grass and tree pollen, dust mites, animal dander, nuts, fruits, shellfish, eggs, cows’ milk, insect bites and stings, certain medication, latex, mould and household chemicals. For most people these allergens are generally harmless, however for people who are allergic to them they can cause all sorts of symptoms.
The most common symptoms of allergic reactions are sneezing, a runny or blocked nose, wheezing, coughing, a rash, and red, itchy eyes. These allergy symptoms can vary in degree of severity, and severe reactions called anaphylaxis or anaphylactic shock can also occur. Severe reactions are a medical emergency and need urgent treatment as they can be life threatening reactions.
Allergies have plagued humanity for millennia. Chinese doctors described a plant fever that caused runny noses in autumn three thousand years ago, there is evidence that the Egyptian pharaoh Menes died from the sting of a wasp in 2641BC, and around 100 years BC the Roman philosopher Lucretius wrote, “What is food to one is to others bitter poison.”
As the existence of allergies has been known for millennia, one might think that the actual cause of allergic reactions is known. However, scientists have not yet been able to find a definitive answer. The process is known but the reason behind that process is not.
The process starts when an allergen lands on one of the body’s surfaces, which is loaded with immune cells that act as border sentries. When an immune cell encounters an allergen, it engulfs and demolishes it, then decorates its own outer surface with fragments of the allergen. Next, it finds its way to some lymph tissue where it passes on the fragments to other immune cells which then produce antibodies known as immunoglobin E, or IgE. When these antibodies encounter the allergen again they activate mast cells, which blast out a barrage of chemicals which trigger the symptoms of allergic reactions, such as itchiness and coughing.
There are two leading theories that scientists are currently exploring for why this happens. Since the 1980s, the leading theory has suggested that allergies are a misfiring of a defence against parasitic worms. This theory is based on research by Bridget Ogilvie who discovered, in 1964, that rats infected with parasitic worms produced large amounts of what is now known as IgE, which signalled the immune system to unleash a damaging assault on the worms.
Having a strong reaction to parasitic worms can be explained by evolution, as they represent a serious health threat. Before modern public health and food safety systems, our ancestors faced lifelong struggles against worms. And even today, more than 20% of all people on Earth carry parasitic worms, though mostly in low-income countries.
The worm theory states that the proteins of parasitic worms are similar in shape to other molecules we regularly encounter in our lives. If we encounter those molecules, we mount a pointless defence, to something that looks similar to a health threat but isn’t really. This would make an allergy an unfortunate side-effect of the body’s defence against non-existent parasitic worms.
For a long time the worm theory was the dominant theory but without concrete proof other theories have popped up and gained popularity. One of these is the home-alarm system theory, based on research by Ruslan Medzhitov, which states that it is not the shape of allergens which explains them but what they do, what damage they cause. From an evolutionary standpoint, the home-alarm theory is easier to explain than the worm-theory, as damaging chemicals have been around for millennia and allergies would have protected our ancestors by flushing them out.
Medzhitov published his theory in 2012 and has since been testing it. So far his research has proved that the immune system responds not to the allergen itself or its shape but to the damage it does. In this it is similar to a home alarm system. “You can detect a burglar, not by recognising his face, but by a broken window,” said Medzhitov and once you have detected a burglar you do what you can to expel him. Much like your body when it reacts to an allergen, as all the major symptoms of allergic reactions have to do with expulsion.
In this theory, allergic reactions are a positive symptom that protect our health. Because the two theories see allergies in a different way they lead to different treatment options. For the worm theory a complete block of allergies is the best option, whereas for the home alarm system theory completely blocking allergic defences is a bad idea, as the problem isn’t the protective response but that for some people the protective response becomes a hypersensitive one.