for chapter 11 we are going to be talking about immunity quite a long chapter by the way this one um so let's dive right into it there is no clear way to define what immunity is you know if you go online or if you check different books um you can actually see that different uh people Define immunity uh you know in various ways so but long story short immunity do not have to memorize this one immunity is just basically your body's ability to fight off pathogens now I don't mean to scare you especially if you're quite an anxious person or you are quite germ phobic okay because some of my students are uh but you have to realize that we are surrounded by pathogens every single time there are pathogens on your skin there's pathogen on the table heck there are some pathogens in your body just waiting to cause an infection but they can't um so these pathogens are constantly harassing and Haring you they're just waiting to all they're trying to do is go into your body and steal your resources but by stealing your resources they're also going to cause damage in your body but here's the interesting thing even though we are constantly surrounded by pathogens in most cases we don't fall sick all the time and even if we do fall sick we usually recover without any permanent side effects there are exceptions obviously okay but under normal circumstances uh the reason why we don't fall sick all the time or even if we fall sick we recover it is usually due to our defense system and what exactly are these defense systems so we have two types of defense system which are the external defense system and the internal defense system for a levels the main one that you have to know is obviously your skin because your skin provides a waterproof barrier and it just prevents pathogens from entering directly into your body simple as that okay and the skin does some extra things uh like you know they produce a certain type of acid to make the pH of the skin about 5.5 so it prevents the bacteria from easily growing sweat also contains certain types of chemicals that can prevent bacteria from uh proliferating so these are just some examples but for a levels uh just in the exam if they ask you to list out an example of external defense system just say skin that's good now another example of the external defense system is in your stomach now I know what you might be thinking the stomach is inside your body should so shouldn't it be considered internal the reason why the stomach is considered external is because the inner part of our stomach is exposed directly to the outside world for example uh notice the purple color thing that I'm highlighting any food that you eat is exposed to the outside wall it goes into your esophagus and directly goes into your stomach so your stomach is exposed to the outer elements so to speak so in that case what actually happens is your stomach produces uh something known as stomach acids or Hydrochloric acids and this acid what it does is it usually denatures the bacterial pathogens and kills the bacteria simple as that so just put stomach acid if they ask you to list out um external defense system another example of external defense system is in our Airways and our lungs yes the lungs are also inside your body but for the same reason as the stomach the lungs are also deemed to be exposed to the outside world because when you breathe air the air from the outer part of your uh body will enter your lungs directly so your lungs are exposed to a lot of harmful pathogens in the air they are exposed to dust they are also exposed to like you know certain types of chemicals but we are focusing on pathogens here so in your Airways such as your track here um there are two very important things such as mucus produced by the goblet cells that trap the pathogens and you also have ciliated cells which contain Celia and they will push the mucus upwards both your lungs are protected from the pathogens in this case now I know that mucus and ciliated cells are supposed to be in chapter N9 I have not done the video on that and I've also mentioned it before in the comment section somewhere that I'm a bit lazy to do chapter 9 if I'm being honest with you um I will eventually get around to doing it but for the time being okay just anyway so moving on uh we have uh the internal defense system now what what is considered internal then internal is what happens when the pathogens for example let's say the pathogens manage to survive the stomach acid and it goes into your intestine from the intestine it enters your blood all the pathogens were in the lungs and then from the lungs they enter the Alvi and from the Alvi they enter the blood so this is now considered inter because when the pathogens are inside your blood vessel so anytime the pathogen is in your blood vessel that is internal and from the blood vessels they can spread to other parts of the body and diffuse into the other parts of the body that is called internal so when they are when when the pathogens are in your blood or in your body cells how then do we actually deal with this infections that is when we need to use the immune system okay and the immune immune system is basically your white blood cells so your white blood cells are part of your immune system I want you to understand something important the immune system is an extremely complicated system very very very complicated Cambridge a levels barely scratches the surface of what the immune system does all right uh and I speak from experience because when I studied immune system in you know school it was like oh white Bloods but when I studied it in University it was like damn it's a lot more than what I expected your immune system goes beyond just white blood cells okay there are also a lot of different types of chemicals that are involved in this in a levels we are going to be looking at two types of white blood cells which are the phocytes and lymphocytes but the true immune system does not just involve these two white blood cells there are many different players in this extremely complicated modality