Immunology Wars Episode I: Physical Barriers & Innate Immunity Explained

In this episode of Immunology Wars Episode I The Unseen Threat we will be looking at some of the physical barriers that our body uses to protect itself and prevent a pathogen, such as a bacteria or virus from entering our body and the

innate immune system which forms the frontline of the body's immune response if a pathogen does manage to breach these physical barriers. So, in Star Wars, the Rebel Alliance is engaged in a never-ending battle with

the Imperial Army, which swarms across the galaxy. We can think of this as a constant battle between our immune system and disease causing organisms that are seeking to colonise our body to take advantage of the favourable environment

it provides for their survival and replication. Just as the Imperial Army is constantly looking for weakness in the Rebel Army's defences in order to invade a planet and harvest the resources for their own benefit. To protect themselves

against attack the Rebel Army have employed a range of defences and these can include physical force field barriers, which represent our skin, deep sticky swamps, which are the mucous membranes that line the openings of our

body, such as the nose and throat, hostile terrain that the Imperial troops struggle to cross, akin to the low pH of our stomach that can destroy pathogens and a network of laser defences, which are the antimicrobial peptides, such as those

present in our tears that can destroy an invader before they enter our body. These physical defences are normally extremely successful in defending the planet and repelling Imperial bombardment, which is our body defending

against the constant presence of pathogens. Sometimes, however, these physical defences weaken and the enemy thinks they have spotted a small gap in the forcefield

that they can sneak through undetected. Just as if we have a cut in our skin it provides a potential entry point for pathogens. Once inside the body the Imperial troops will attempt to quickly set up a secret base from which they can

launch their attack, in the same way, that a bacteria or virus will enter one of our own cells in order to replicate. Fortunately for the Rebel Army one of the planet's resident Ewoks, Warrick WWickett, spots the intruder and helps sound

the alarm. Here the Ewok represents a mast cell which is often found in our tissues that plays an important role in the defence against pathogens. The Ewok sends up a distress flare, which

alerts the Rebel Army to the presence of the invader and helps clear a path to this invader so that the first Rebel troops can reach the battle more easily. In our body, this distress flare is a chemical called histamine, which dilates

blood vessels, increasing blood flow to the site of injury or infection which causes the redness and swelling, which we associate with inflammation and this allows cells of the innate immune system to be directed to the area more

easily. Han Solo is the first troop to arrive on the scene and as he usually does he rushes into the battle with all guns blazing to destroy the Imperial troops, leaving a trail of bodies in his wake.

Solo represents a type of immune cell called a neutrophil, this is the most abundant cell in our immune system and forms the frontline defence against disease-causing organisms including bacteria, fungi and viruses.

The reason neutrophils are so abundant is that they can only destroy one pathogen or infected cell before they themselves commit suicide. The pus that sometimes is present at an infection site is actually the decaying bodies of

these dead neutrophils. As a result of this neutrophils can often become overwhelmed by the sheer number of pathogens, just as Han Solo often rushing into the battle finds himself in a sticky situation, surrounded

by Imperial troops. Fortunately for Solo,l his loyal sidekick, the normally placid Chewbacca, becomes alerted to his friend's fate and leaves his post to enter the battle as a marauding killer Wookie, helping destroy

many of the Imperial troops left behind. Chewie represents another type of immune cell called a monocyte, which normally circulates in our blood, but when recruited to the tissue at the site of infection or inflammation becomes a

cell known as a macrophage. Unlike the neutrophil, which can only kill one pathogen, the macrophage is able to destroy many pathogens or infected cells. However, even Chewie is unable to stem the seemingly endless droid army

and calls in an airstrike from the Rebel Army central command post to carpet-bomb the area and destroy any remaining Imperial troops. This represents a systemic fever that can sometimes accompany infection, which is controlled

by the hypothalamus, a small area of our brain, responsible for trying to maintain or restore balance, also known as homeostasis. Amongst the charred remains of the battlefield the Jedi Master Mace Windu,

who was visiting the planet, spots a bounty hunter, Jango Fett, who appears to have been corrupted by the dark side as he is no longer wearing his identifier tag. Windu acts swiftly, wielding his lightsaber to poke holes in the bounty

hunter resulting in his death. Windu represents a type of cell called a natural killer cell, which is probably the best name for an immune cell whose main job is to destroy cells that are infected by a virus or which are

potentially cancerous. The natural killer cell does this by looking for the presence of a molecule called MHC class 1 on the surface of our cells and if this identifier tag is missing releases a chemical called perforin, which

punches holes in the target cell causing its death. Following the battle the cleanup operation begins with planetary residents and members of the Rebel Army repairing their homes and fixing

gaps in the force field in an attempt to prevent the return of the Imperial troops. This is a repair and resolution phase of the immune response, where any damaged tissue is removed or repaired and the

body restores balance or homeostasis. So, in this episode, we've briefly looked at the physical barriers and the innate immune system defending against a pathogen invading our body. In the next

episode, Attack of the Clones, we will take a closer look at the adaptive immune response which is a more specialised response involving the immune system learning to identify and destroy specific threats.

I hope you'll join us then.