C3.2 Defence Against Disease [IB Biology SL/HL]

this is the video for the standard level portion of C 3.2 on defense against diseases disease is a bit of a broad term it could be um from environmental causes genetic causes or infectious causes and in this video we're going to be focusing mainly on infectious diseases and those are diseases that can be passed from one organism to the other so that's the key term here is this passing from one organism to the next infectious diseases are caused by things called pathogens and pathogens can be organisms or viruses and that includes things like bacteria fungi

protest and of course viruses we'll be learning about a lot of sophisticated mechanisms for dealing with pathogens but our body's first line of defense its primary defense mechanism involves two things that we don't often think about about in terms of immunity which is our skin and our mucus membranes so our skin is actually made out of um a very tough barrier and this outer layer of skin is actually dead so the skin that I'm touching right now is made up of dead cells and because those are dead cells it forms a really kind of like

impenetrable barrier for pathogens to make their way into our bloodstream so that's how pathogens um become very bad and dangerous when they get into our bloodstream skin prevents that we obviously have all kinds of holes in our skin right holes that we need but holes nonetheless and that's an opportunity for pathogens to get into our bodies so around those openings to the skin we have what's called a mucus membrane so anywhere there's an opening in our skin so eyes ears mouth nose vagina you um anus urethra all of those are covered by mucus membranes and

that is a membrane that produces this really sticky mucus that traps the pathogens so almost like a fly trap so that those pathogens don't make its way into our bloodstream now let's say I poke a hole in my skin I get a cut in my skin that can be really bad for two reasons one I don't want to bleed to death but two I don't want to create an opportunity for pathogens to make their way into my bloodstream I need to form a clot really quickly to plug up that hole and so this whole sequence

of events called a clotting Cascade is going to happen after we get a cut in our skin and here's how that works first we're going to have these little things called platelets attached to that cut platelets are tiny cell fragments okay and once they attach to that site where the cut or the hole in the skin is they are going to release chemicals called clotting factors there are lots of different clotting factors it's a whole family of chemicals but in general these clotting factors are going to cause prothombin to be converted into thrombin so prothombin

is a chemical um that's in our blood and it's kind of in an inactive form the clotting factors convert it to its active form called thrombin and then thrombin starts to act like an enzyme this thrombin converts fibrinogen into fibrin there's a lot of conversions here so fibrinogen is a soluble blood protein so it's floating around it's it's dissolved in our blood plasma it's produced in our liver and when it comes in contact with thrombin thrombin converts it to fibin and fibin is insoluble so it comes out of solution and it forms this mesh that

we see here this mesh is going to trap um blood cells and platelets things that are floating by in the bloodstream and this trapped blood cell platelet fibrin mesh is the clot okay and so this is what forms a blood clot to seal those holes in the skin so when we think about overall holistically how our body defends itself against pathogens we've got a couple of different options we just talked about that primary defense system which is our skin and mucous membranes that's going to be different than our immune system okay so immunity is not

the same thing as that primary defense and even within that branch of immunity we have two types of immunity so there is an innate immune system and an Adaptive immune system innate immune systems involve these cells called fagos sites site means cell fago refers to phagocytosis this engulfing process okay and this remains constant throughout an organism's life it is not specific so these are not uh cells that attack specific pathogens they attack anything that doesn't belong there the Adaptive immune system involves cells called lymphocytes again site means cell lympo these are going to be cells

living in your lymph nodes and these are going to be the cells that build that immunity build that memory throughout an organism's life so this becomes more and more intricate as an organism gets older they are specific to different pathogens so I'm going to have different immune U mechanisms for different pathogens let's do a quick review of the types of blood cells so we'll be talking about two types of blood cells although there are many so the first of which being erthrocytes erthrocytes are also our red blood cells they're for carrying oxygen primarily so we

won't be discussing them further in terms of immune system then we have white blood cells also known as lucco sites again there are lots of types of lucco sites we'll focus on two broad categories of lucco sites one of which being fos sites so fagos sites are going to be um like these macr phases or fosic lucos sites they're going to gobble up they're going to engulf um pathogens then we have lymphocytes so lymphocytes are going to be um part of adaptive immunity okay so these are going to be producing specific or geared towards the

production of specific antibodies fagos cytes are nonspecific Within These lymphocytes we'll talk about two different types of cells okay so we're going to have te- cells te- cells are responsible for identifying the correct B cells that can produce an antib body that is specific to the antigen on a pathogen B cells when the correct one is found these are going to clone themselves many times via mitosis it's something called clonal selection some of these B cells are going to different differentiate into plasma cells plasma cells are going to produce the antibodies other B cells will

differentiate into memory cells and these are the basis of immunity so they'll remain in our bloodstream to produce uh antibodies for an infection later on let's chat about these phagocytes again these are part of our innate immunity they are nonspecific and we're going to hear them referred to by a few different terms possibly fosite maccrage and fosic lucsy those terms are all synonymous they're all referring to the same type of cell and they are again nonspecific that's important and what we mean when they say non-specific they're not just running around like gobbling up anything they

can what they're doing is they're reading those antigens those recognition proteins on the outside of a cell and they are determining whether or not that thing uh belongs there is it self or not self if this macras if this fagos site does not recognize what it's reading if it categorizes it as nonself then it is going to engulf that pathogen it's going to identify it as a pathogen engulf it via endocytosis now once that pathogen is on the inside of the cell we can see that happening here okay that this pathogen is going to be

engulfed and now it's sitting inside of a vesicle inside of this fosite once that happens inside enzymes inside lomes I'll try to draw one here a lome full of enzyme will merge with that vesicle full of pathogens and it will destroy the pathogen okay so this fosy this innate immune cell is there to destroy pathogens and it does that by engulfing them and then using the enzymes inside of its Li now when it comes to adaptive immunity what we want to associate adaptive immunity with is antibodies a for adaptive a for antibodies and those are

going to come from all of those lymphocytes that we talked about now we talked about lots of different types of cells T cells B cells plasma cells memory cells there's a whole system of cells that have to cooperate in order to produce those antibodies okay so we'll C from the beginning lymphocytes are cells that are found in the lymph nodes that's how we get that word and they circulate throughout the bloodstream they produce antibodies antibodies are y-shaped proteins so they kind of look like this a y-shaped protein and on the end of the protein there's

a special shape I should have done a better job here of drawing this on the end of this y-shaped protein um there's a special shape a special binding site that will match with the antigen on the outside of a pathogen so again that antigen is a recognition protein on the outside of a cell or a virus and these antibodies have specially shaped um bits to match up with the those antigens um on this pathogen so the way that these antibodies work is once they bind to the pathogen they either tag that pathogen for Destruction by

other immune cells that we will not talk about or what this can do is it can prevent this pathogen from using its antigen to bind and to and infect other cells so the way the antibodies work is a little bit um complicated and not really the focus of this part this is really more focused on where do these antibodies come from and why do we need so many different types so again antibodies are specific so I'm going to need a different antibody for every antigen and so that means I need to be able to make

different types of lymphocytes each lymphocyte is only going to be able to make a very limited uh number or types of antibodies so if I need lots of antibodies because there are lots of antigens that also means I need lots of different lymphocytes So within my lymph nodes I'm going to have thousands of different versions of these lymphocytes all ready and waiting to make these different antibodies um for the different antigens on the pathogens now antigens aren't just on pathogens they're recognition proteins those glycoproteins that we're going to find on the surface of all cells

and viruses so for example I have type B blood if you've already studied blood groups this is so interesting here the connection my type B blood um has Type B antigens along the outside okay so my immune system recognizes those that thinks that those are great that they belong to me it also means that my immune system is going to produce antibodies called anti-a antibodies these are going to be antibodies that are going to have um a fit to the type A antigens okay so what will happen if type A blood cells are injected into

my bloodstream okay is that my immune system will produce these anti- a antigens it will bind to these cells because it does not recognize these itself and I will have an immune response so this is why we need to make sure that if we're getting a blood transfusion it's only the blood cells um that match our blood group that we're doing that and that's again because antigens that our body does not recognize as self will stimulate an immune response next we're going to talk about a series of steps in our adaptive immune system and that

means that everything should be leading up to the production of antibodies so the first thing that we're going to see is the appearance of this macras or fosy okay so this is the first cell that comes into play in our little film about antibodies here and that pathogen is going to get engulfed by those fagos sites so it's going to engulf the pathogen and then destroy it using the enzymes in its lomes and then it does something really weird it takes that destroyed pathogen and it wears it on its outside like a hat or a

costume and that's called antigen presentation it's almost like it's saying hey guys look what I found I'm wearing this dead pathogen what should we do so once that antigen presentation has taken place the next cell to be activated is what we call the te- cell or helper te- cell or tea lymphocyte okay again a te- cell that is a lymphocyte that's what we're looking at now that antigen presentation on the outside of the fosy will activate that t- cell and the te- cell's job is to find the specific B cell that can make that antibody

so remember B cells are also an example of a lymphocyte so you might hear them referred to as B cell or B lymphocyte I know this is confusing but hang in there okay so the activated te- cell is going to locate the correct B cell that is capable of making the antibody that we need in order to fight that pathogen once that B cell has been identified the te- cell's job is done and now we're going to hand it over to that specific B cell that B cell is going to clone itself many times this

is something called clonal selection now that we have found and activated the right B cell we need a lot of them so lots of mitosis there then it's going to start to differentiate okay so that differentiation process is going to turn some of these B cells into plasma cells so when we say differentiate that means that they're going to express different genes and one of the genes that they're going to be expressing is the ability to produce antibodies so they're going to grow and they're going to produce organel for antibody production remember antibodies are proteins

so that means they're going to be building a lot of rough endoplasmic reticulum and a lot of gges and this is where the antibodies are coming from so all of this was in order to secrete lots and lots of antibodies this idea of memory cells is a higher level topic so we'll leave that for now but again the whole goal here if we're thinking about antibody production really required a lot of interaction and interdepend attendance between different types of cells within our immune system