hi everybody and welcome back today we're going to be looking at immunity now in this video we are going to cover the entire immune system reaction moving from the first line of defense all the way to the third line of defense and it's important to know that um when we speak about immunity there is more than one kind of immunity and so essentially what we're going to focus a lot on is your acquired immunity and essentially what that is is the immunity that you have developed during the course of your life now you get passive immunity which is a type of immunity where someone or something else has provided you with the defense against the infection and that's what immunity is all about providing protection against foreign substances or pathogens from entering the body now a natural way to get passive immunity would be through the transmission of like antibodies from a mother to a baby via breast milk and i want you to know that passive immunity is very short term it's not long it doesn't last forever on the other side you can also get artificial passive immunity which is when you are given a serum medicine often what happens is for example with tetanus people who have stepped on a rusty nail are exposed possibly to tetanus and instead of giving them a vaccine we give them a serum of antibodies which allows them to fight off any kind of infection that may already be there now passive immunity is short-lived and um it doesn't require actual exposure to the pathogen in order to develop immune response in other words someone else is giving you antibodies the one we're more familiar with is active immunity and that is when you are exposed to an infection and um you develop a response to it you can also use active immunity for when you get a vaccine and so there's two ways that this can happen the first way the natural way is let's say you're exposed to some kind of pathogen like a virus may have perhaps something like chickenpox um you then develop antibodies for chickenpox and the next time that you're exposed to it your body can fight the infection better and faster the artificial way is the vaccine way and there are multiple kinds of vaccines available today but essentially what happens is you're injected with some form of the pathogen whether it be a dead pathogen or a piece of the pathogen and your body remembers what that pathogen looks like so the next time if you are exposed you don't fall ill so in order to be a successful immune system you need multiple lines of defense and our first line of defense is non-specific in other words it doesn't target a specific pathogen it doesn't prevent a certain kind of bacteria or certain kind of virus from entering essentially it can't tell the difference between what's infecting you instead it is the protection that we have on the surface of our bodies and so this means it's not in the tissues and it's not in the bloodstream in other words these are the things that you have on the surface of your body that prevent substances from going into your tissue or going into your bloodstream now in the picture here we have some examples um some non-specific surface immunity would be something like your eyes and the tears that you have your tears have antimicrobial proteins in them which prevents bacterial growth you also have in your respiratory tract and in your nose you have a mucous lining now the mucous lining is really important because often pathogens get stuck in the mucus themselves and they can't actually get into your bloodstream or into the tissue and lastly the most important one in terms of the first line of defense because it's the biggest one is your skin now your skin is a physical barrier but it also has chemical barriers as well and that is for example sweat now sweat changes the ph of your skin it makes your skin a lot more um unhospitable to pathogens for them to grow and so often what you find is that the surface of your skin is a little more acidic and that's because bacteria and fungi they struggle to grow in a more acidic environment and that's what you want to maintain as your first line of defense but what happens if a substance like a pathogen gets beyond your skin and that brings us to the second line of defense so our second line of defense is a midway between being in the tissue but not actually being in the actual blood and so if you look at this photo this alongside this is exactly what i mean what we're looking at here is we have a wound of some kind and something has pierced the skin here perhaps it is a nail or something sharp even a splinter and you'll notice that it's not in the bloodstream so the bloodstream hasn't been broken there's no blood you're not bleeding think of something perhaps like like getting a splinter in your skin you're not bleeding but you still pierce the skin so we're in the soft tissues now and a very common uh associated with the second line of defense is what we call the inflammation response now the inflammation response literally means swelling and fever and it's a non-specific response and remember non-specific means that it doesn't matter if this is a virus or a bacteria it will affect you the same now there are other things associated with inflammation besides swelling and fever and that's also some antimicrobial proteins which i'll discuss now where essentially these are substances that draw out the white blood cells and bring them closer to the infection so let's say for example in our photo here we have a splinter stuck in the surface of our skin what does our body do to prevent any of the pathogens that are on that little piece of wood or in that little piece of splinter what is our body doing so what you notice here is we've made an incision now into the skin and you'll see that on the surface there were bacterial cells now what happens is when you start an inflammation response there are these tiny little antimicrobial proteins which is what you can see here it's these red um circular structures and those are the antimicrobial proteins that are released by the skin to warn your immune system that there is something in the skin and in the soft tissue that is possibly um carrying a disease and so what does that do it attracts these really large white blood cells which we call phagocytes and they are attracted to the area and what they do is they squeeze through the spaces in the in the capillaries they make their way into the soft tissue and then they digest and destroy any foreign pathogens now if we were to look at what happens in terms of the order of events once you have damaged the tissue you are going to produce a chemical called a histamine and i'm sure you've heard of medication called antihistamines which essentially are prescription medications that prevent swelling inflammation and if you have hay fever that's also what you can take it for and so what a histamine does is it induces swelling it induces heat and so what that does is it also attracts the phagocytes to this area and it makes your blood vessels swell and it makes them more permeable now you want that because number one you want the swelling because the swelling leads to more blood flow to the area more blood flow to the area means more white blood cells and the second thing is you want the permeability because if the skin is damaged you want more blood to rush to the area and you want the white blood cells to be able to squeeze through the empty spaces and get into the bacterial cell now how do these big white phagocytes destroy your pathogens well they do this through phagocytosis so let's zoom in on what exactly is phagocytosis so phagocytosis essentially is when a phagocyte which is a really large white blood cell is going to engulf some kind of foreign particle in this picture here we have a bacterial cell and so what happens is the phagocyte will approach the bacterial cell and it will engulf it in other words it will trap it inside its cytoplasm so it moves towards it and it surrounds it then what happens is that your phagocyte is going to isolate it on its inside it's going to keep it there it's going to make almost like a membrane around it to make sure that the bacterial cell can't escape and it can't be left behind anywhere and then finally what the phagocyte does is it uses its lysosomes now if you remember what a lysosome is it is a temporary vacuole that contains digestive enzymes and those digestive enzymes will degrade the bacterial cell break it down into its smallest components make it essentially harmless and once the phagocyte is done with that it can absorb anything if it wants to keep it but it can also remove any excess pieces of the bacteria cell as essentially cell waste and it can be excreted out of the phagocyte via exocytosis now both of the lines of defense we've just done are non-specific so it doesn't matter what infects you this is how your body responds but what happens if it's a very specific pathogen a specific virus and what does your body do if it's now in the bloodstream because up until this point this is all occurred in this on the surface of your skin or in the soft tissue but not actually in the bloodstream well now we're going to look at the third line of defense so the third line of defense is comprised by very very specific cells previously we had non-specific responses and this is a very specific response and there are two kinds of cells involved in the third line of defense the first one are our t lymphocytes now our t lymphocytes come in three varieties that we need to know of and the first kind is what we call a helper t cell also known as cd4 cells and these are really important because what they do is they begin your immune response and so for example if you have an infection that's now made its way into your bloodstream these cells the cd4 or helper tees they are going to be the alarm bell ringers they're going to say to the body we are infected we need help and besides calling other t lymphocytes which i'll get to now they also call the help of b lymphocytes now we'll get to b lymphocytes but b lymphocytes are the antibody producers so we have two assistants happening at the same time we have the helper tees calling for help from other white blood cells and they are also calling help from the b lymphocytes now the second thing that can happen is um we can also call for help from what we call killer t cells now killeties as their name suggests destroy infected cells but this is important they destroy host cells in other words they actually destroy your body's cells the ones that are infected though you see the problem is that once a virus is inside of your cell your body can't actually tell that that cell is sick you need a special cell to identify that that tissue that cell over there is infected because viruses hide inside your cells and your body doesn't know that it shouldn't be there and so when we speak about killities i want you to know that they target specifically viruses only or they can also target your own cancer cells because they're going to be the ones looking out for um cells that aren't behaving like they should and that's what killer teas do and so to have a look on the left hand side here of how t-cell activation works let's have a look at exactly how the response begins and so first of all we have a virus now viruses if you remember are going to live inside your cells they spend a very short period of time outside of your cells so now we have a problem the cell is infected now remember your body doesn't necessarily know immediately that that cell is infected so luckily we have t cells now these are the cd4 cells that are doing this so this is your cd4 cell and the cd4 cell is starting the immune response it's calling for help how does it do that well that's what these cytokines are it's basically a chemical message to say hi we need help please come to this area and so now we've activated the t cells now the t cells undergo replication and an option of that replication is to produce and to call killer teas and so what you have here is a whole bunch of killer tea tea lymphocytes showing up and what they're going to do is they essentially feel around and they use their external proteins these little pieces that stick out the side here to determine whether or not a substance is a foreign particle or a foreign or infected cell and so what they actually do is they they touch the surface of a cell and they go hmm this doesn't look right there's something inside of here that's changed this cell's chemistry and what the killeties will do is they will destroy that infected cell then we can cue the phagocytes the phagocytes as we know are almost like the cleanup crew they'll go into an area they'll absorb damaged particles they'll absorb pathogens and they'll remove it from your system now something that i want to explain as well is well then how do we know what does and doesn't belong in the body well there are substances called antigens and essentially what they are is on the surface of every single cell within your body there is the cell membrane and sitting on the cell membrane will be some antigens and basically they're like these little proteins that sits on the outside of your cell that say i belong here i am from this body and through these little antigens that sit on the surface your body knows okay and these cells belong here this is the shape of the body's antigen we know that this is not a foreign invader but now here's the problem if the foreign invaders antigens don't match our own that's when the body recognizes it as foreign so let's say for example that this cell that i've drawn here is our own cells let's say we had a virus cell infect us but now its antigens on the outside are a different shape and that's how the body tells that that foreign invader is a foreigner it knows what our antigens look like and because they are not the same and they don't have the same shape as you can see this is what our antigen looks like this is what the virus's antigen looks like because they don't match one another you will see a response to it and that is how if we look over at our diagram again that is how the t cell knows whether or not it's a foreign invader you can see that it's touching this infected cell and when it touches the infected cell it notices that there's something wrong with the antigen what that actually notices is that if i put one of the viruses um antigens inside our own this little triangle it notices that that's different that that shouldn't be there and now the killer t's know that these cells are infected by this virus and so that's how we know another way we know is that if this virus that was just floating around inside of us if it was this one over here if it just floated around phagocytes can also tell the difference but unfortunately phagocytes have to touch the outer area first to know if it's a foreign invader now the third kind of t lymphocyte that you can get is a really important one and it's there because you eventually have to stop the immune response when you are getting better these cells are known as the suppressor cells and they are type of t lymphocyte that effectively does exactly as the same name suggests they stop the response and that is because your immune system can actually be negative to you if it continues because sometimes what happens if immune response continues um it can drain you of resources it can make you weak it can essentially allow you to get sick again by another infection so you want to eventually stop the immune response we finally need to look at b lymphocytes now b lymphocytes are the cells that are called to assist the immune response and they are called if you remember by our cd4 cells our helper cells and those helper cells call the b lymphocytes in and what happens is the b lymphocytes can recognize an infection via the antigens remember antigens of the proteins on the outside of the cell membrane that tell your immune system yes i belong here or no i don't belong here and how beta versions are different to t lymphocytes is that b lymphocytes can destroy both viruses and bacteria whereas killer teas can only destroy viruses inside of the host cell and also kill it is remembered to cancer now b lymphocytes on the other hand well they have the ability to do viruses and bacteria and why and how they do this is they essentially transform into a super producing very um specific response cell called a plasma cell which i'll explain now what that is or some of those individuals will become a memory cell and as the name suggests it is the way in which our body remembers infections so let's go through the step-by-step process with the diagram about how b cells are activated so let's say for example there's a virus it has now infected a healthy cell and it can also be floating around in the bloodstream so doesn't have to be inside of something and this is what bacteria would do the bacteria would be floating out in the open and what happens is the cd4 cells the helper t's have called for assistance and the b cells show up and what they do is they determine by looking at the antigens on the surface of the cell or the surface of the bacteria that that pathogen doesn't belong there and now it knows these are viruses or these are bacteria i need to remove them and so what happens is they send out an alert which are those cytokines it's basically a chemical message and it alerts all the other b lymphocytes that were already on their way to activate so in other words it's like whoever gets there first and because city falls have called them to come and see we're now sending out a response to all the b lymphocytes saying there's a virus here there's a bacteria here this is what it looks like and now you need to respond and so we've got two options the first option is um you make a plasma cell so that's this option down here it's not labeled i'm just going to write here plasma and so a plasma cell is a really important cell because it is a type of b lymphocyte and so we started off as a b and now we've turned into what we call a plasma cell a plasma cell is the cell that makes antibodies so our plasma cells have shown up that's step one and they've now identified that there is a foreign invader and they are going to start producing these antibodies that we've spoken about now these antibodies must match the antigen of the pathogen remember those are the structures on the outside and what exactly is an antibody well an antibody is a protein and they generally come in a y shaped configuration so the protein looks like the letter y and essentially that one antibody can act in multiple ways to destroy the infection and to make it easier for the infection to go away and if that's the antibody then if this was our bacterial cell or our back uh or our virus on the outside there are its own antigens and if you remember our body is not going to be able to tell that those are ours it's gonna it's going to be able to say that's a foreigner we need to get rid of it and so what happens the antibodies they can fit in on this space and they label it as a foreign invader now this area that they connect into they are going to remember it in future so they're going to remember this shape over here the next time there is an infection but let's go through how antibodies actually destroy a foreign invader so antibodies are going to do some of these following things first of all what they're going to do is they are going to burst the bacterial cell so the antibodies can get stuck on the outside of the bacteria it makes the cell wall very weak and the bacteria bursts it can also make it easier for phagocytes to find them so it's almost like a like a label um to say hey immune system these bacterial cells are not supposed to be here i'm like a little label it'll flag to say we need to remove you another thing that antibodies can do is they clump pathogens together and this just makes it easier like a cleanup if you want to get rid of everything you want to sweep everything together and make it all stick together and so antibodies can do that and so antibodies stick to each other and they clump the pathogens together making it easy for the phagocytes to come in and just engulf and destroy a big collection of them and lastly what they can also do is they neutralize toxins now certain bacteria produce a toxic chemical and when your body needs to respond to that they're not just responding to the bacteria they must also respond to the chemical that's being made and that's ultimately probably going to make you sick and maybe even lead to death so antibodies don't just respond to a physical and bacterial cell viral cell it can also respond to a substance that they make now once we've made the antibodies and we're destroying the the invader we must remember the invader for the next time that this could happen and that's the second option so in activation we can also make b memory cells and these are your immunological memory cells they're going to remember the infection the next time and these are the vital ones for when we get vaccines when we get a vaccine we are essentially doing this whole process but with a weakened virus or a weakened bacteria and essentially we want to remember it the next time and so what happens is our body makes antibodies it makes memory cells and those memory cells will remember that infection for the next time you may be exposed so to summarize that our memory cells maintain the memory of the pathogens antigen and so remember every pathogen has a specific antigen it's like a shape of the protein and so that the next time affection occurs we have a faster response now i don't know if you know how long responses are but generally they can take from anywhere from five to seven days before there's a full response and by that time you're feeling very very sick if however you have had this disease before or you've had a vaccine you often won't even feel any symptoms of being exposed because your body will have the memory cell and it will automatically start making the antibodies so we don't have to wait for the cd4s and we don't have to wait for the b lymphocytes to show up identify the foreign invader then turn into a plasma cell then make antibodies we just literally go straight from memory cell identifies the foreign invader makes the antibodies and we can see that if we look at the diagram here is our plasma cell producing our antibodies those antibodies get stuck on the outside of our infection in this case it's a virus but it can be a bacteria as well and you can see the cleanup crew the phagocytes they move in and they engulf and destroy any left over cells materials that need to be removed so let's do a final terminology recap so pathogens pathogens are substances that cause diseases think virus bacteria protest or fungi now you have lines of defense in your body your first line of defense is mostly made up of your skin and your mucous membranes remember that's a physical barrier it's non-specific and it protects you on a variety of substances getting inside of you but what happens if the substance gets into your skin but not in your bloodstream and that brings us to the second line of defense you can have inflammation which is swelling and fever and you also make your blood vessels more permeable to blood and white blood cells moving around and you can also produce antimicrobial proteins now these proteins can be secreted onto the surface of the skin or it can be secreted in the actual soft tissue and an important substance that calls um the immune system to your skin and to make sure that nothing's moving into the bloodstream are called histamines and histamines attract phagocytes and phagocytes essentially destroy any pathogens either the actual cell or bits and pieces of the remains of their cell structure then we looked at specific immunity which is the third line of defense and we have helper teas who alert the body that you have been infected we have killeties who destroy viruses and cancerous cells and remember these killer t's specifically target host cells so in other words they kill your cells that are infected and then we went on to b lymphocytes and b lymphocytes can produce something called a plasma cell plasma cells then produce antibodies and antibodies match up with a specific pathogen and that pathogens antigen will tell us they belong or not and last but not least memory cells memory cells are cells that we keep for sometimes months and some we keep for years and our entire life and they will remember the infection so that the next time if we are exposed the immune response is faster i hope that you found this video useful and i will see you all again soon [Music] bye