In this video, we're going to begin our lesson on a process known as clonal selection. And so first, we need to recall from some of our previous lesson videos that each individual B and T cell actually has many identical BCRs or TCRs on its surface that only allows it to respond to one very specific antigen. However, within our bodies, there are pre-existing populations of many different B and T cells that have different BCRs and TCRs. And collectively looking at all of those B and T cells within those populations, they can recognize and respond to a wide range of many different antigens. So when we do get infected with a very specific antigen, how is it that our adaptive immune system ensures the proliferation of only very specific B and T cells that can appropriately respond to the given infection that we have? Well, in order to answer that question, we have to take a look at the clonal selection theory. And so the clonal selection theory basically states that upon being infected with a very specific antigen, only very specific B and T cells within a mixed population of many different types of B and T cells are going to be selected to become activated and make identical clones of itself. And so in other words, what we're saying is that the clonal selection theory says that only the very specific B and T cells that bind the antigen or respond to the antigen are going to be selected to proliferate and create an army of clones that are effective towards that very specific antigen that we are infected with. And so what this means is that all the other B and T cells that do not bind the very specific antigen that we are infected with, those are not going to be selected to proliferate. And so only the very specific B and T cells within the populations of B and T cells are going to be selected to proliferate and respond. And so we can get a better understanding of the clonal selection theory by taking a look at our example down below of the clonal selection theory where we apply it to B cells. But again, it also applies to T cells as well. And so here we're looking at the clonal selection of very specific B cells within a mixed population of different B cells in response to a very specific antigen. And so notice if in this image we have 3 different layers. We have this top layer here, this middle layer right here, and then we have the bottom layer at the bottom. And so at the very top here in this first layer, what we're showing you is the mixed B cell population. And so what you'll notice is that within our bodies, we'll have a population of many different B cells. So here we have B cell number 1, B cell number 2, and B cell number 3, and these are 3 different B cells. Now within the same B cell, the BCRs will be identical and respond to the same antigen. However, across different B cells, those B cells will have different BCRs that allow them to respond to different antigens. And so notice that here in this image, the antigen that is present is this red antigen that you see here. And notice that not all of the B cells will respond to this very specific antigen. However, the B cell that does recognize and respond to that specific antigen will be selected to move on to the next stage, which is B cell activation and proliferation. And again, proliferation just means to multiply and create identical clones. And so notice that it is only B cell number 2 that has the BCRs to bind and respond to this very specific antigen. And so it's B cell number 2, not B cell number 1, not B cell number 3, that is going to be selected to proliferate. And so notice that this B cell becomes activated and it begins proliferating or making clones, dividing. And so now we have an army of clones that are capable of responding to this very specific antigen that we are infected with. So then after B cell activation and proliferation, of course, what we have next is the differentiation of the activated B cells. And of course, that means that it's going to be changing its phenotype to become either a plasma cell that secretes antibodies or a memory B cell that will respond to a secondary infection of this same exact antigen. And so notice here that we have this activated B cell differentiating into these plasma cells, which will again secrete antibodies, produce antibodies. And these antibodies will be specific to the very specific antigen that we are infected with, helping to eliminate that particular antigen. And, again, notice that some of the B cells, instead of differentiating into plasma cells, they will differentiate into memory B cells, and they'll be able to respond even faster upon a secondary infection. And so here, this here concludes our brief lesson on the clonal selection theory and how only very specific clones within a mixed population are going to be selected to proliferate and differentiate in order to respond to the very specific antigen that is present. And these other B and T cells—the other B cells that are here—they will not proliferate unless their very specific antigen is present. And so, again, this concludes our brief lesson on clonal selection, and we'll be able to get some practice applying these concepts as we move forward. So I'll see you all in our next video.