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Many of us have heard tales of the notorious “peanut reaction”: The sufferer upon consuming food laced with peanut traces starts gasping for breath and if not treated promptly with a jab from an EpiPen, soon collapses and may even lose their life. This feared condition is known as anaphylaxis. It is a severe body-wide allergic reaction triggered by the mass-release of immune chemicals in response to an allergen. How does it work? Anaphylaxis can be triggered by any of a variety of allergens – from food like shellfish and peanuts, to dust and pollen, to insect stings and more. But let’s continue with our example of peanuts…

A Fateful First Encounter

When Person X’s body comes into contact with peanuts for the first time, their immune system mistakenly recognizes a certain protein from the peanut as harmful. The reasons for this are still under research, but some possibilities are thought to be genetic make-up, lack of exposure during the critical period of immune system maturation when the person’s immune system is learning to differentiate the harmful from the harmless, or a combination of these.The encounter could be something as insignificant as eating food with peanut traces left behind during processing, breathing in peanut flour or oil droplets suspended in the air, etc. And, the body’s response is usually so subtle that Person X may not even show obvious signs of having an allergic reaction! Immune cells called B lymphocytes make small amounts of antibodies against the peanut protein. These antibodies remain floating around within the bloodstream and tissues of the body (keep this in mind for later!). The immune system also “saves” the chemical signature of the peanut protein to its “database” by forming memory cells, which will recognize it in future encounters and activate the immune response.

The Catastrophic Confrontation

Alas, the next time Person X’s body comes into contact with peanuts, their immune system reacts in full force. There are two waves of response.

The First Wave

Remember the antibodies against the peanut protein floating around within the bloodstream and body tissues? Some get attached to the surface of immune cells called mast cells. In the first wave, peanut protein binds to these antibodies, triggering the mast cells to release a potent cocktail of immune chemicals like histamines, prostaglandins and leukotrienes. These chemicals spread throughout the body via the bloodstream and kickstart a chain-reaction of changes.

Peanut protein binds to antibodies on the surface of a mast cell. This triggers the mast cell to release immune chemicals which then spread throughout the body via the bloodstream.
Illustration by © Jacy Mok

Well, histamines, prostaglandins and leukotrienes are also released in response to invading bacteria and viruses. They help our immune cells reach the affected site via our bloodstream more quickly and squeeze past the blood vessel wall into our tissues more easily. These relate to their action on blood vessels – in particular, blood vessels widening and becoming leakier. However, in the case of anaphylaxis, the mass-release of these immune chemicals results in a battery of collateral effects on the rest of the body.Within minutes to an hour, symptoms strike! These mostly relate to blood vessels widening and becoming leakier. The skin may become flushed. Itchy, red-rimmed bumps (i.e. urticaria) may appear. There could be swelling of the eyes, lips and/or tongue. If even the tissues along their throat and airways swell up, Person X will develop difficulty in breathing. Blood pressure may drop dangerously, and Person X could faint with reduced blood supply to important organs like the brain. They could also experience an upset gut, with abdominal cramps, vomiting and/or diarrhoea.

The possible effects and symptoms of anaphylaxis.
Illustration by © Jacy Mok

When two or more body systems are affected, Person X is considered to have anaphylaxis. It is an emergency and requires immediate medical attention! Yes, please call an ambulance.Media and pop culture have also raised our awareness about the EpiPen, which with a jab to the middle of the outer thigh, administers a fast-acting and potentially life-saving dose of the hormone adrenaline. It makes the heart pump faster, helping to maintain the blood pressure. It narrows blood vessels to less important parts of the body like the skin and gut so that blood flow is diverted to more important organs like the heart and brain. It helps to widen the swollen and narrowed airways, improving air flow into and out of the lungs.

Adrenaline is also administered by doctors in the hospital, either intramuscularly (i.e. via injection into muscle) or in rare cases, intravenously (i.e. directly into the bloodstream through a vein). In addition, the patient may be given supplemental oxygen if they have difficulty breathing, intravenous fluids to keep their blood pressure stable, as well as an intramuscular or intravenous anti-histamine to counteract the flood of histamines released by their immune system (remember that potent cocktail of immune chemicals mentioned earlier?).

Illustration by Jarrod Chua

In severe cases where the patient cannot get enough oxygen from the air into their lungs by themselves, a tube will be inserted into their windpipe, either through the mouth or as a last resort, via a slit made in their throat. This tube is connected to a breathing machine and allows air to get past their swollen tongue and throat tissues so that it can reach their lungs.

The Second Wave

Meanwhile, the rest of immune cells are not just idling around. The immune response continues beyond the initial burst of mast cell activity. Chemical signals named cytokines are like battle cries that call reinforcements to join the fray. These include macrophages, eosinophils and T helper lymphocytes, which each release their own set of immune chemicals, recruit more immune cells, and/or present fragments of peanut protein to activate them. This is the second, more delayed wave of immune activity.

Chemical signals called cytokines attract macrophages, eosinophils and T helper lymphocytes. These in turn release their own set of immune chemicals and recruit more immune cells.
Illustration by © Jacy Mok

And, remember those memory cells formed during Person’s X first encounter with peanuts? Some of them are memory T lymphocytes, which are rapidly activated once they encounter the peanut protein and amplify the immune response. Others are memory B lymphocytes, which start churning out large numbers of antibodies against the peanut protein. Some of these antibodies stick to the surface of yet more mast cells, leading to further bursts of mast cell activity and immune chemical release.

Memory B lymphocytes churn out large numbers of antibodies against the peanut protein. Some of these antibodies stick to the surface of mast cells.
Illustration by © Jacy Mok

This is why even after initial emergency treatment, the patient with anaphylaxis is warded for observation for at least 24 hours, and given repeated doses of intravenous anti-histamines and steroids at intervals to dampen their immune response.

A Word of Reassurance

Fortunately, not all allergies have as far-reaching or fatal effects. Most affect only a single system or part of our body. Examples are common rashes like urticaria and eczema, as well as the red eyes and runny nose some of us get whenever we come into contact with dust or pollen, also known as allergic rhino-conjunctivitis. Symptoms can often be controlled by avoidance of the allergen, or treated with a short course of anti-histamines and/or steroids if they interfere with daily life or should the allergen be unavoidable or unidentifiable.In the context of allergies and anaphylaxis, our immune system can at best seem annoying and at worst be terrifying. But let’s remember that ultimately, it is our body’s most important line of defence against invaders like bacteria and viruses. So, let’s be thankful that it exists and not throw the baby out with the bathwater.

Written by Jacy Mok


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