The Science of Allergies

Spring is now upon us and along with trees and flowers, seasonal allergies begin to bloom. It’s estimated that 21 million people in the U.K. suffer from allergies, and this may include you. But what exactly are allergies? What causes them? And can they be treated?

Common symptoms of allergies include: sneezing, runny nose, cough, wheezing, swollen lips. Image: Sid, Source

What are allergies?

Allergic reactions begin in our immune system. This amazing biological warfare unit is normally fantastic at identifying and destroying foreign invaders such as harmful bacteria and viruses in our bodies. But sometimes it can make mistakes. Some people have a hypersensitive immune system that reacts to otherwise harmless substances such as dust or pollen, resulting in an allergic reaction. There are many external stimulants, or “allergens” that can trigger allergic reactions – airborne particles like animal hair, insect bites or stings, or ingested material like nuts and certain medicines.

What causes allergies?

Once in our body, how do these allergens actually cause an allergic reaction? The answer lies with lymphocytes. These cells are an essential part of the immune system and help to guard our body against foreign substances such as invading bacteria and viruses. They move freely through our body, ensuring that no harmful substances are present. They’re like the customs officer of our body, making sure only cells with the correct passport are there.

There are two types of lymphocytes – B-cells and T-cells. When a B-cell encounters a foreign substance in your body, such as an allergen, it responds by producing antibodies against it. Antibodies are “Y-shaped” molecules that attach to specific types of allergens. Think of them like a lock and key. Each antibody is a key that can only attach to one specific allergen, or lock.

There are 5 basic types of antibodies, called immunoglobulins, or Igs:

  • IgA
  • IgD
  • IgE
  • IgG
  • IgM

The antibody responsible for allergic reactions is IgE.

The first time someone is exposed to an allergen, they won’t actually experience an allergic reaction to it. The allergen is just recognised by B-cells, and they then produce IgE antibodies against it. IgE then goes on a hunt for another type of immune cell found in our bodies, the mast cell. Mast cells are found in all of our body, but are mostly concentrated near sites of allergic reactions, such as the nose, throat, skin and lungs.

IgE attaches to these mast cells, allowing them to recognise the foreign molecule when it next enters the body. The mast cell acts as a security guard, while the allergen-specific antibody as a mug-shot. The mug-shot allows the security guard to quickly recognise the criminal when they reappear. Similarly, next time the allergen enters the body, the mast cell will be able to immediately recognise it and respond. Mast cells attack the foreign molecule by releasing a barrage of toxic chemicals including histamines. Histamines cause many of the symptoms we commonly associate with allergic reactions – for example they can constrict airways, making it difficult to breathe. But the exact reaction depends on the type and amount of allergen encountered, and of course, the person affected.

Screen Shot 2017-03-23 at 14.42.36
The allergic cascade. B cells recognise the allergen and secrete antibodies against it. These antibodies bind to mast cells, weaponising them for when the allergen next reappears. When the mast cells encounter the allergen they release lots of chemicals, causing symptoms such as watery eyes, itching and a runny nose. Illustrated by Yasmin of “Weird and Wonderful Science”.

This is why someone who has been stung by a bee before might only become allergic to the bee venom on a second encounter.

In rare cases, an allergy can lead to a severe allergic reaction, where the patient experiences hives, swelling of the tongue and tightening of the throat. This is called anaphylaxis or an anaphylactic shock and can be life-threatening.

Currently, the only effective treatment of this is an injection of epinephrine. Epinephrine is a hormone that counteracts the severe effects of anaphylaxis by re-opening the airways.

Can allergies be treated?

“The best way to treat allergies is to avoid the allergen altogether.”

In terms of treatment, most doctors recommend steering clear of the allergen. Of course, this is not always possible. Allergens such as dust and pollen are nearly impossible to avoid, no matter how clean your house is. Allergies such as these can usually be managed by medication such as antihistamines and decongestants.

Another potential treatment is immunotherapy, and this is the closest thing to a cure for allergies. But it can be expensive, time-consuming and risky. This therapy involves a series of injections of the offending allergens, starting off with a very weak dilution but gradually increasing the strength to a “maintenance dose” that is then continued over time. The injections effectively help the immune system to produce fewer IgE antibodies, and thus prevent an allergic reaction occurring.

Maybe sometime in the future we will be able to develop a permanent cure for these pestering allergies, but until then just avoid running through pollen-filled fields full of bees. Oh, and nuts. And dust. And horses…

Image: Razvan Orendovici, Source
Header image: Darla, Source

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