Illustrated by Rachel Ko

It’s a fine Saturday afternoon. You’re sitting in your backyard sipping on coffee and losing your mind over the daily Wordle. While you’re so engrossed, an unusual, blue-colored creature pulls another chair and solves the Wordle for you. Just as you look up and try to process the condescending smirk of this creature, your daily news notification pops up. It's true! The whole world has been invaded by aliens!

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Thankfully this is a figment of our imagination, but would you believe me if I told you that alien invasions are constantly happening unnoticed in the microscopic world of our bodies? Every day, our cells face new ‘alien invasions’, thanks to unhygienic eating, or even just from breathing! 

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In the external world, such an invasion would unsettle the entire human population and adversely impact the lives of everyone. It’s amazing how such invasions inside our bodies are usually defeated daily. So who are these tiny ‘soldiers’ that fight them off, silently and efficiently?

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It’s time to introduce the two brothers of our story– the innate immune cells system and the adaptive immune cells system, the former being the more enthusiastic and energetic one, while the latter is calmer and wiser. Although different in nature, the two systems coordinate efficiently to eliminate our enemies and help us go on about our lives.

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The innate immune system acts first when a pathogen (a disease-causing microorganism) manages to enter our bodies by getting around our physical barriers like the skin, and the mucus in the respiratory, gastric, urinary, and sexual tracts, etc. The innate immune system consists of cells like macrophages and dendritic cells (DCs), which are constantly looking out for incoming invaders. These cells recognise pathogens through common foreign attributes that our native cells don’t possess. In order to defend us from the harmful effects of the pathogen, our innate cells engulf them. In fact, the word ‘macrophages’ literally means ‘big eaters.’ Inside our cells, the pathogens’ end is inevitable, smashed and broken into pieces, which are mounted on our soldier cells’ surfaces, informing other soldier cells that an invasion has occurred. 

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Exposing broken parts of the pathogen on our innate cells’ surfaces also produces chemicals called cytokines that help recruit more of our soldier cells to the site of invasion. So, when we get flu, the secreted cytokines is why we run a fever, cough, sneeze, and influx of our soldier cells to the throat area is why we may have swelling around there. Similarly, if we bruise, our blood vessels dilate to allow entry of our soldier cells to the wounded area, which is then manifested as redness and swelling around it. Fortunately, this means of communication of our soldier cells is much faster than our internet connection and so the whole process occurs in a matter of hours.

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On most days, the keen innate immune system is enough to control an invasion. However, it needs big brotherly advice from the adaptive immune system in case things get out of hand. The main players of this part of the immune system are the calm B- and T-cells. These can be found resting in the lymph nodes, unaware of the invasion in the body.  

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The B- and T-cells are wise soldiers, which is evident in the way they respond to an invasion. Each of these cells has molecules called ‘receptors’, which uniquely recognise pathogen parts presented to them. These receptors, on an adaptive cell, can be thought of as padlocks and the broken pathogen parts, mounted on an innate cell, as a key. In the lymph nodes, each resting B- and T-cell has a different type of padlock, unique for a different key. It is the job of a DC, with a broken pathogen part mounted on its surface, to enter the lymph nodes and search for the most accurate match for its key, from the variety of B- and T-cell padlocks. The key varies based on the different types of pathogens that invade our bodies. Once the perfect match is found, that specific B- and T-cell is activated and rapidly multiplied. This lock-and-key method of activation of adaptive cells confers the specificity of their action.

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These activated cells move from the lymph nodes to the site of infection and perform different functions that halt the pathogen from spreading the disease, by either killing the pathogen or stopping its reproduction. At the site of infection, innate cells, with the key (broken pathogen part) mounted on their surface wait for the brotherly advice, the incoming adaptive cells with the perfect match to the key. The activated T-cells uniquely interact with macrophages and signal them to start killing the pathogens that they have engulfed. This helps with clearance of the pathogen.

Although B-cells are part of the adaptive immune system, they can also recognise the foreign pathogen products, break them down, and present these parts on their surface, just like the innate immune cells. So now B-cells also have a key to the activated T-cell padlocks. Their lock-and-key interaction facilitates the B-cells to release antibodies. Finally, the antibodies, together with the macrophages and DCs, as well as the B- and T-cells of the adaptive immune system, successfully win the war and die peacefully, having completed their purpose. But a small portion of B- and T-cells go on and develop into long-lived memory cells. Over the span of our lives, we are infected and reinfected with pathogens all the time, however not every encounter results in us falling sick. The credit goes to the B- and T-memory cells and their ability to remember the foreign attributes of the pathogen and kill it as soon as it re-invades.

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Adaptive cells’ memory is the principle of vaccination. An inactive pathogen or a part of the pathogen is introduced into the body. This trains our soldier cells for a real pathogen invasion by triggering the B-cells to form memory and specialised antibodies against the pseudo-pathogen. If the real pathogen infects us again, these pre-formed antibodies make fighting the war much easier and quicker.

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Correct training of immune cells is essential since a pathogen invasion is a life-or-death situation for us. Any mistakes by our soldier cells can have devastating effects. For example, an important part of the training process is to ensure the immune cells aptly distinguish between civilian cells and foreign cells. This education occurs in the bone marrow. Here, any B- or T-cells that attack civilian cells or cell parts are evicted from the training process so only the most eligible soldier cells continue to become eligible soldiers. (1)

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But even after a rigorous selection process, things can go wrong with our immune system. Instead of being our defending heroes, they turn their back against us and start identifying civilian cells as aliens and attacking them.

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Sadly, this is the reality for 5% of the Australian population, with a majority being women. This condition, when the immune cells stop distinguishing internal cells from alien cells, is called an auto-immune disorder. The cause for this disorder is mostly unknown, with some speculations of it being genetic or environmental. The repercussions can be mild, such as causing dry mouth and dry eyes - symptoms for Sjogren’s syndrome, or more severe such as joint pain and immobilisation, known as Rheumatoid Arthritis. These diseases are currently life-long and incurable because they involve our own cells fighting the healthy cells in our body. (2)

Nevertheless, the immune system plays a very important role in helping us lead normal lives. It fights the battle against the invaders daily, without us realizing it. Thanks to the soldiers of the immune system, our daily activities, like solving a Wordle on a relaxing Saturday, are not hindered by an alien cell invasion in our bodies! 

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References

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1. Kenneth Murphy, Casey Weaver. Basic concepts in Immunology. Janeway’s Immunobiology. 9th ed. United States: Garland Science Taylor and Francis; 2017. p. 4-11

2.  Overview of autoimmune diseases [Internet]. Healthdirect. Available from: Overview of autoimmune diseases | healthdirect