hello everyone welcome back to a.s biology with dr demi i am dr demi and in today's video i will be going through lymphocytes so if you didn't watch the preceding video which is chapter 11.1 immunity phagocytosis please make sure you watch that first because it goes into detail of how your immune cells respond to an infection and this is simply a continuation of that in that video we spoke about how there are two types of immune cells so it is important for you to please have a look at that video before continuing with this one if however your main aim is simply to understand lymphocytes then you are in the right place let's get into it okay so lymphocytes first of all they are smaller than phagocytes so if you watch the previous video you would remember that the phagocytes are a kind of immune cells lymphocytes also have a large nucleus and for the sake of this syllabus we are going to focus on two types of lymphocytes those are your b lymphocytes and your t lymphocytes or you can refer to them as the b cells and the t cells the b cells stay in the bone marrow until they are mature and then they spread throughout the body concentrating mainly in the lymph nodes and in the spleen the t cells on the other hand will leave the bone marrow and go to the thymus which is where they mature always remember that only mature cells can carry out immune responses now when i ask students in class where is the thymus students erroneously refer to their thyroid so this image from cancer help uk is a very important image for you to sort of to help you visualize where the thymus is so if you look over here there is the thyroid over there i'm just going to use my red pen again here this is your thyroid so it is somewhere in your throat the thymus on the other hand is closer to your respiratory organs um so the thymus and the thyroid are two different organelles or two different organs within the body so please do not confuse them that is the point of having this image over here okay so lymphocytes just like we said when they mature they circulate between the blood and the lymph remember when we discussed transport in animals we said that we have a circulatory system which is how blood is transported around our body but we also have what we call a lymphatic system and the lymph is simply liquid that has been forced out of the capillaries probably due to high pressure and circulates differently helps to get rid of excess proteins and all of those things so if you haven't watched the videos on transport in animals please again make sure you do so so that this is not confusing information for you something else that's interesting about the lymphocytes is that they deliberately distribute themselves around the body so that they can encounter pathogens so just like the neutrophils in phagocytosis or the neutrophils which are a kind of phagocytes the lymphocytes also tend to move around the body trying to find any pathogen that might be invading the body the lymphocytes are considered to be the actual immune response of the body simply because of how they act and we are going to see that in a bit of detail very soon all right so the b lymphocytes or the b cells as you might want to call them are the first ones we would look at when b cells start to grow and uh this is something that i think would be a great assignment for you is to find how b cells develop and then grow but as they start to mature they gain the ability to make one type of antibody molecule which means that different b cells make different antibody molecules this means that the b cells are highly specialized so if we have b cell number one b cell number one is probably designed to make an antibody molecule that looks like something that fits with the influenza virus which causes the flu and b cell number two would have an antibody molecule that fits into some other kind of infection so the antibodies that are produced by b cells are part of the immune responses if you've lived through the covid19 pandemic you're probably aware that some people who were infected with covered produced antibodies against covid and at some point this became a plausible treatment technique to say if we harvest some of these antibodies we can reproduce them and then use them to treat people who are sick so antibodies are very very important um the different b cells so let's say for example you get infected by a particular type of pathogen and the b cells are able to respond to make an antibody once they're able to make these antibodies they then multiply into a small number of cells that produce the same types of antibodies in other words the b cells as they mature and they start to become more aware of the pathogens that invade your body they clone themselves so we call them clones of each other so let's again go up to our example of b cell number one b cell number one makes an antibody that addresses influenza this b cell number one will learn how to make the antibody that fits onto influenza and then it would clone itself so it clones itself to make many more types of itself so that that group of cells would now then be called the group of b1 cells and they would all make antibodies that fit with the influenza virus please have a look at your books and just try to learn about how b cells grow and mature and this might be more clear to you if it's not clear still please feel free to post a comment and i will address it as soon as i can b cells can also become plasma cells which produce several thousands of antibodies very quickly so plasma cells will secrete antibodies into the blood the lymph into the gut as well but they die very quickly these plasma cells die quickly the antibodies that they produce would stay for longer some b cells also become memory cells which means that they are able to remember an antigen so i often tell my students how often do you think you get infected or i often ask them rather how often do you think you get infected with the colds the common cold or the flu and many of them would say uh i only get sick once a year or i never get sick at all and i said well it's a good thing you don't get sick but it doesn't mean you were not infected so sometimes you get an infection but because your b cells have learned what that pathogen looks like these are the memory b cells they are able to produce antibodies very quickly and destroy the infection before you even develop any symptoms so when you go about your daily life you you know hanging out with friends you're hugging people who are sick you're probably getting infected with a whole lot of infections but because these are infections that the b cells the memory b cells have sinned before they don't allow any symptoms to occur instead they simply say we've seen this before we've made these antibodies before so let's just kick off production immediately this means they don't have to learn about the antigen again from scratch they simply know what it looks like they release antibodies and as a result the antigen is destroyed before it is able to cause any kind of symptoms in the infected person so something to bear in mind here with b cells is that b cells can become plasma cells plasma cells release thousands of antibodies into the blood or into the lungs or the gut and plasma cells die very quickly but the antibodies that they release stay for longer some b cells can also become memory cells which then enable them to remember any antigen that they've encountered before and to prevent symptoms from developing if you get infected with that same antigen again with that said we then need to look at the t lymphocytes so the t cells happen to play a very important role in eliciting an immune response and in the t cells there are three different types but for your level we only look at two we look at the helper t cells and we also look at the killer t cells so t cells on their surface if you look at these images over here let me just see if i have my red pen on or rather let's use blue to them if you look at these images over here they have all of these receptors on their surface these receptors are very similar to antibodies and also they're specific to antigens so when t cells encounter a foreign antigen that they're specific to what they do is that they divide through mitosis to increase their numbers so when they increase their numbers that then causes an immune response of sorts which i would explain in detail but something interesting i want you to look at here is this cell over here that's um this receptor here that's called cd4 this cd4 is really important because it is what the hiv virus actually binds to in order to um invade the t cells and to destroy them so that they are unable to elicit an immune response against it it's very interesting i often advise students to go and do some research on it just to see how exciting it is when we're talking about immunity and how the body fights all of these viruses and infections and also what makes hiv such a smart virus so the helper t cells they release hormones right so once the helper t cell sense that oh goodness there's an antigen within the body the first thing that they would do is release hormones and these hormones will stimulate the right types of b cells to divide and secrete antibodies so let's say again using the cold virus or let's just say influenza as as a as an example so let's say the helper t cells are going around the blood and they find an influenza virus in the blood what they would do is that they quickly release cytokines these are chemicals that would stimulate the b cells those cytokines then send out a signal remember when we discussed phagocytosis we were speaking about how histamine is released by cells that are affected by by an antigen and that causes neutrophils to gather around the region in this case think of the helper t cells they're trying to help start the immune response so they release cytokines and as a result of that the b cells um are stimulated now not just any b cell is stimulated it has to be the b cell that is specific to that antigen especially if you've seen the antigen before so let's say it's the influenza virus which is something that we all tend to encounter very frequently then the b cells that are responsible for antibodies that fit influenza work very quickly to release antibodies so the antibodies will then go there and they would bind to that influenza as a means to keep it in check until the macrophages can come and destroy it some helper t cells stimulate macrophages which then conduct phagocytosis very aggressively which means that they engulf very quickly and digest whatever it is that has invaded the body you also have the killer t cells now the killer t cells are cells that simply kill they are trained to kill and what they do is that they search the body for cells that have been invaded by pathogens so sometimes some pathogens are really smart they get to a particular cell that's unsuspecting they knock on the door the cell lets them in and so they hide in there and they try to use the cell to reproduce themselves or to just grow themselves what the killer t cells do is that they patrol these kinds of cells or patrol the body looking for these kinds of cells and when they find any foreign antigens they will attach themselves to the surface of the infected cells and secrete harmful substances to kill that cell which means they're simply saying this cell is no longer loyal to us it is now working with the enemy so it has to be destroyed so killer t cells will then destroy that cell and as a result destroy the pathogen that infected it killer t cells also um help a t cell sorry can also stimulate the rapid division of killer t cells so when you think of helper t cells always think of them as cells that simply help to elicit an immune response and that is why when they get attacked by hiv they are unable to send out a signal to say we have been attacked or there's something foreign in the body and as a result of that hiv is able to ravage all of the body cells and insert itself and make its viral proteins now when you have an infection something that's very important for you to know is that the number of neutrophils that you have will increase when you have a bacterial infection or when you have an inflammation so remember that neutrophils when they fight an infection they die at the site of the infection and they collect as pus so if you have a pimple sometimes a pimple might be due to some kind of inflammation or maybe bacteria has gotten under your skin and it has caused a bit of swelling and the neutrophils would go there to fight it and then they die and come out as pus when you have a viral infection or you have tuberculosis the number of lymphocytes will increase the hiv virus like i've been saying specifically attacks t cells the helper t cells and it destroys them because once they are destroyed again they are unable to stimulate the immune system to say we being invaded something is going wrong and as a result of that hiv is able to spread itself this is why when hiv patients are being monitored they usually check for the number of t cells in the blood because the more t cells a hiv patient has it means that the virus is not progressing as it probably could so that means that that person has a higher chance of getting through the disease however if the t cells decrease significantly that shows that the virus is making way or making headway and destroying the cells that are able to elicit an immune response and as a result of that the person is likely to progress and would be open to other opportunistic infections like tuberculosis and other things that can then result in death to close off i just want to speak a little bit about stem cells and leukemia now many of our white blood cells come from our stem cells we have myeloid um stem cells that make our neutrophils monocytes and they also make platelets and we also have our lymphoid stem cells that produce the lymphocytes now if a person happens to have leukemia which is cancer of these stem cells what will happen is that the cells will divide very rapidly and because they do so it means that they are not able to mature so they don't differentiate as they should and that affects the number of normal blood cells that the person would have so you might find that a person who has leukemia is unable to produce enough red blood cells or even enough platelets and some people are unable to produce enough neutrophils simply because the cells are not allowed to mature properly so that is just something to bear in mind um there's usually questions are usually questions about this um in the exams but not very detailed but i would suggest that you do some deep reading about this just so you're not caught on a ways as you prepare for the exam thank you for watching this video in the next one i will discuss active and passive immunity i will also make mention of what we call the primary and secondary immune responses because that's a very important question in the exams and then we will try to have a look at vaccines which is a very big debate now in the time of covet so i hope that you find that very interesting have a good time goodbye