[Music] so what we're going to be looking at right now is the T lymphocytes and just a little bit of revision to T lymphocytes um I mentioned it in a previous video that they are formed in the bone marrow but they matture in a particular organ known as the thyers that is why we refer to these particular cells as T lymphocytes the first important thing that you have to know about these particular types of lymphocytes is there are two types of tea lymphocytes which are known as t- helper lymphocytes and T killer lymphocytes sometimes in the exam they may refer to them as te helper cells and te- killer cells they mean the same thing you can use both terms in the exam you'll be fine now the main thing to know about the T helper lymphocytes and the T killer lymphocytes they also have receptors complementary to an ens just like B lymphocytes if you remember in the previous video I said to you that b lymphocytes have their receptors um on their cell surface membrane and they will react accordingly to complementary antigens or matching antigens so if tea helper cells and te killer cells also have receptors um what makes them different well the obvious answer is their immune response or how they react to antigen so what we're going to do is we're going to look at T killer lymphocytes first what are the functions of the T killer lymphocytes when I ask my students this question they will be they will look at the word killer and they will assume ah what the lymphocytes do is they will kill pathogens directly so my students will say that the T killer lymphocytes will directly meet with the pathogen where their receptors are complimentary with the antigens and then they will immediately kill the pathogen you know simple as that but here's the weird thing that is not exactly what happens the te killer lymphocytes do not kill the pathogens directly what they're supposed to do is they are tasked to kill our own body cells then you might be thinking why in the world would my te killer lymphocytes kill my own body cells that does not make any sense well let's look at a situation here I'm drawing out a tissue a group of similar cells let's say let's say as an example these are like you know some skin cells and you know the skin cells are grouped together so they form something known as a tissue all right now imagine for a second that one of the skin cell became mutated what exactly is mutation it is when the gene under goes a random change in the base sequence and sometimes when Gene under goes uh random changes in the base sequence they form something called onco This is chapter 5 by the way and when they form onco gen the cell becomes a cancer cell and what's the characteristic of cancer cells they may uncontrollably divide and when they divide they might press on the other cells and they might cause a tissue damage as we have discussed this in chapter 5 uh sometimes what may happen is the pathogen May infect your cells and when they infect your cell your cell is forced to produce more pathogens within the cytoplasm and the pathogens May spread to other cells and we don't want this to happen so we have an infected cell and we have a cancer cell here and they are now a problem so the function of the tular lymphocytic cells in your body or even any cancer cells in your body by the way hence the term killer they don't kill pathogens directly that's not what they do they their function is to patrol around our body around our tissues and look for infected cells and cancer cells and if they find these infected cells and cancer cells their function is to kill those cells which are technically your own cells by the way all right but by killing these cells it prevents the other tissue the other cells in your tissue from being damaged so that's a good thing now here's the thing that is very important that we have to know how does the killer cell know which cell to kill because you can't just give the te killer lymphocyte you know go wild okay just go kill as many cells as possible you don't want that to happen because that will cause damage to your body um so the tea killer cell needs to know which cells are infected or which cells are cancerous uh so we are just going to focus on the infected cells for this time okay so for example over here you can see nine cells which are forming a tissue and the pathogen infects one of the body cell and when it infects one of the body cell which I've highlighted in red okay you can see the pathogens in the cell they are proliferating or reproducing within the cell the infected cell does something extremely important the in Ed cell knows that it has no chance of survival and if this process continues the pathogen May spread to the other cells so what it does is the infected cell will present the antigens on its cell surface membrane so when it presents the antigens on the cell surface membrane this is known as antigen presentation so why is this important because when the tular lymphocytes are nearby look at the four tular lymphocytes that I'm throwing over here will all four of them respond to the cell no because each T killer lymphocytic shaped receptor so the first tular receptor on the tular cell is complementary to the antigen that is presented this process is known as same thing clonal selection we saw this with B lymphocytes as well okay so the similarity is the clonal selection happens as per usual with the antigen and the tular lymphocytes are stimulated and when they are stimulated they will undergo clonal expansion just like B lymphocytes by the way and when they undergo clonal expansion they will form something known as a grp group of activated tular cells and a group of memory teiller cells all right the difference with B lymphocytes formed plasma cells and memory B lymphocytes but here it becomes activated tular cells and memory tular cells the function of the activated tular cell look at it it binds to the cell with the complementary antigens and it releases chemicals into the cell and when it releases cytotoxic chemicals you don't have to memorize that term cytotoxic but it's good to understand it the teiller cells will release something called cytotoxic chemicals and they these chemicals will kill off your infected cells and that's a good thing so as you can see when it kills off the cell it also destroys the pathogens within the cell so it prevents the pathogens from spreading to other parts of the body and in this case over here when the cell dies the the cell under goes a process known as apoptosis and then your fyes will just come and Munch it all away okay but you don't have to know that in detail don't worry about that uh and the other cells are safe okay so what the activated teiller cells will do is the they will Patrol the tissue to remove all the other infected cells that are present in the area and of course what's the function of the memory tular cells in the future they will have they will produce a faster resp response towards infected body cells and they will eradicate any infected cells that you may have in your body simple as that this is what we have to know about the function of the te killer lymphocytes they are supposed to react only to infected cells because the infected cells will present the antigens on the cell on the cell surface membrane it under goes clonal selection due to the complimentary receptors of the T lymphocytes the T lymphocytes or the te killer cells will undergo clonal expansion and form activated tular cells and memory tular cells and the activated tular cells will basically destroy any infected cells in the vicinity keeping your other cells safe