In this video, we're going to take a look at the human body's defense system and how it protects us against pathogens. We can split the system between the physical and chemical barriers, which stop pathogens from getting in, and our immune system, which destroys any pathogens that do make it into our bodies. Let's start with the physical and chemical barriers. If you haven't noticed, most of our body is covered in skin, which acts as a physical barrier that the pathogens can't get through. As well as just being there, it also secretes oils and antimicrobial substances, which can kill the pathogens that rest on it. So, if pathogens can't get through our skin, then they're going to have to enter through holes like the nose or the mouth. To try and prevent this, our noses are adapted by having lots of little hairs and lots of mucus, which can trap the pathogens and any other particles as they try to flow through in the air. If they manage to get through this defense, then they're going to have to go through either the trachea or the esophagus because those are the only two tubes leaving our throat. If they head for the lungs via the trachea, then they'll find that the trachea, bronchi, and bronchioles are all covered in a layer of mucus, which again will trap any particles. To prevent this mucus from building up, they're also lined with psyia, which are tiny finger-like structures that protrude from the cells. And as these psyia rhythmically move, they slowly move the mucus and trap particles up to the back of the throat where we can swallow it down to the stomach. Now though, we've got all of this pathogen filled mucus in our stomach along with all of the pathogens that were hiding in our breakfast. Luckily though, our stomach produces hydrochloric acid, which is a really strong acid that brings the pH of the stomach contents down to about two, which is so acidic that it kills pretty much all of the pathogens. Despite this formidable defense system, though, some pathogens will always make it into our bodies. In these cases, our immune system takes over and hunts them down. The most important part of our immune system is the white blood cells which continually patrol the body circulating in our blood and tissues and they have three important functions worth knowing. One of these is fagocytosis which is a process by which certain white blood cells can directly consume or to choose a correct term engulf pathogens. They do this by first tracking the pathogens down, binding to them and then engulfing them at which point they can destroy them. Another function is producing antitoxins. One of the reasons that pathogens cause us so much trouble is that they can produce toxins which are effectively tiny poisons that can damage our cells. In response to this, our white blood cells produce antitoxins, which are small molecules that can bind and counteract these toxins, so they can't do us any damage. The final function we're going to discuss is producing antibodies. Before we do though, I just want to clear up any confusion about the terms antibodies and antigens. Antigens are substances that our immune system detects as being foreign, such as the cell wall of a bacteria or a virus protein. Antibodies, on the other hand, are small proteins made by our own white blood cells that can lock onto these foreign antigens and then act as signals to tell our white blood cells to come and destroy them. Importantly, each of our antibodies is specific for a particular antigen. So if it can bind to this particular antigen, then it won't be able to bind any others. Once our immune system knows which of our antibodies can bind the pathogens though, it starts producing loads of them and it remembers this for next time. So if we're attacked by that pathogen sometime in the future, then our body will start producing so many antibodies so quickly that we'll destroy all the pathogens before they get the chance to develop. Which is what we mean when we say that we're immune to a disease. And that is basically how we survive the millions of pathogens that attack us every single day. If you haven't heard yet, you can find all of our videos on our website, cognito.org. You'll also find questions, flashcards, exam style questions, and pass papers. And we track all of your progress so that you always know what to study next. So, sign up for free by clicking here or browse our playlist here on YouTube.