T cells and B cells become activated when their antigen receptors bind the appropriate antigen. Essentially, these lymphocytes just flow and hang out around the lymphatic system, the blood, the spleen, and some even in the skin and mucosal associated lymphoid tissue until they get a match. It's a kind of scattershot approach that the adaptive immune system takes. They try as many different things as they can, and if they get a hit, that's great. But if cells don't encounter the proper epitope within a certain time frame, they'll actually die. Cells will eventually die. And if they do encounter the right epitope though, they will essentially clone themselves. They'll divide and turn, create what's called a clonal population. Just a big group of cells that are clones of them. This is part of what's called clonal selection theory. Essentially, this is an idea that says antigens will basically select the lymphocytes by the lymphocyte just encountering them and, if they are activated, they will make many clones and those clones will become what are called effector cells and memory cells.
Before we talk about what those are, I do want to point out that sometimes lymphocytes that have not been exposed to an antigen yet are called naive lymphocytes. So that's just a mature lymphocyte that hasn't yet been exposed to its appropriate antigen. Now, effector cells are going to be the short-lived but fast-acting cells of the adaptive immune system. They're going to take immediate action against the pathogens that are present. Memory cells are going to be longer-living cells. They will continue to divide at a low rate to keep up a population and they're going to hang around and be used to fight future infections. So, their job is not to take action against these current pathogens invading. Their job is to wait and see if they come back and then mess those dudes up. Here, you can kind of see an example of how this sort of clonal selection goes down. We have these naive lymphocytes. This particular one has a match. So it's going to clone itself a bunch. These other guys, they didn't get matches so they're going to die out. Sorry. It's brutal in the immune system and this clonal population is going to divide between some memory cells and some effector cells that you can see behind me. Alright, with that let's flip the page.
