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Biology: Immunity: Clonal Selection Theory


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About this Lesson

  • Type: Video Tutorial
  • Length: 11:48
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 127 MB
  • Posted: 07/01/2009

This lesson is part of the following series:

Biology Course (390 lessons, $198.00)
Biology: Animal Systems and Homeostasis (63 lessons, $84.15)
Biology: The Immune System: An Introduction (6 lessons, $9.90)

Taught by Professor George Wolfe, this lesson was selected from a broader, comprehensive course, Biology. This course and others are available from Thinkwell, Inc. The full course can be found at The full course covers evolution, ecology, inorganic and organic chemistry, cell biology, respiration, molecular genetics, photosynthesis, biotechnology, cell reproduction, Mendelian genetics and mutation, population genetics and mutation, animal systems and homeostasis, evolution of life on earth, and plant systems and homeostasis.

George Wolfe brings 30+ years of teaching and curriculum writing experience to Thinkwell Biology. His teaching career started in Zaire, Africa where he taught Biology, Chemistry, Political Economics, and Physical Education in the Peace Corps. Since then, he's taught in the Western NY region, spending the last 20 years in the Rochester City School District where he is the Director of the Loudoun Academy of Science. Besides his teaching career, Mr. Wolfe has also been an Emmy-winning television host, fielding live questions for the PBS/WXXI production of Homework Hotline as well as writing and performing in "Football Physics" segments for the Buffalo Bills and the Discover Channel. His contributions to education have been extensive, serving on multiple advisory boards including the Cornell Institute of Physics Teachers, the Cornell Institute of Biology Teachers and the Harvard-Smithsonian Center for Astrophysics SportSmarts curriculum project. He has authored several publications including "The Nasonia Project", a lab series built around the genetics and behaviors of a parasitic wasp. He has received numerous awards throughout his teaching career including the NSTA Presidential Excellence Award, The National Association of Biology Teachers Outstanding Biology Teacher Award for New York State, The Shell Award for Outstanding Science Educator, and was recently inducted in the National Teaching Hall of Fame.

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Think about the four things you need to have an immune system work properly. Remember, you need to be able to go self versus non-self. You need to be able to know the difference. Your cells need to be able to know the difference. They need to be diverse, they need to be specific, and they need to have some kind of memory.
Well, I want to give you an overview right now of something called the clonal selection theory. In the clonal selection theory we're going to see the essence of immunology. There's plenty of time for details later, but let's take a look at what clonal selection is all about, because it is the basic premise of immunology.
Let's start out with this whole idea of self versus non-self. How does that happen? How can you have a diversity and still manage to not eat your own tissues? And in fact, you know probably that there are some diseases where there is autoimmunity, diseases where you actually attack your own body with your immune cells. More on that at another time.
Let's talk about self versus non-self. Well, here's what happens. Remember this whole idea of B cells and T cells? And you remember that they mature in two different places. There's a cell called a lymphocyte. Lymphocytes are undifferentiated cells that are going to turn into B and T cells. You remember that if they go to the bone marrow, they become B cells, and if they go to the thymus, they become T cells. Now, here's the thing. When you have a B cell or a T cell, you're going to form on that cell--I'll pick either one--you are going to form receptor sites on this thing, on the cell membrane, and that receptor site is going to recognize chemicals. What chemicals are that are foreign to your body are called antigens. So this antigen receptor site, or antigen recognition site, if you will, is going to recognize chemicals, particularly foreign chemicals. Recognizes antigens. But how come it doesn't treat your own cells as an antigen? And it is in the thymus and the bone marrow that these and every single one of these cells will have identical antigen receptors--maybe 100,000 of them on one cell membrane. Kind of an exaggerated diagram--you'll see better diagrams than this later.
So one of these cells will have one that will recognize one specific chemical of all the chemicals possible on the planet literally. Now you're saying, "Wait a minute," you're telling me you have a literally infinite number of these things? Pretty close. I'll tell you how that happens later. So you've got this one B cell and it has all these antigen recognition sites on it, and they're all the same. Maybe this particular B cell might recognize because you have an infinite number--"infinite"--it might recognize one of the proteins that are in my blood. Okay, maybe for some strange reason this might recognize hemoglobin. You don't want a B cell attacking hemoglobin. Guess what happens? In the bone marrow or the thymus each of these cells are routinely exposed to the materials that are normally in your body. And here's what happens. If it recognizes self, if it's one of the ones that recognizes self, it's a reject. And in being a reject, one of two things is going to happen to it. It's going to be deactivated, so it's going to be turned off, or apoptosis is going to occur to it. Programmed cell death. It explodes. They kill it. In this way, your bone marrow and your thymus are eliminating those cells that are going to recognize self in those antigen recognition sites.
Look! We just did self versus non-self. Now we know--there's number one. We've got it. Non-self--self versus non-self--check. Okay, number two. Let's see what happens now. Let's get sick. Let's have somebody walk up to you and germ you. So somebody with like strep throat comes up and coughs all over you, you get bacteria in you, and now your primary immune response, your third line of defense. Your third line of defense, here it comes. It's not an inflammation. You've got things invading your body. Your third line of defense--specific immunity.
So you're germed. What happens? Here's what happens. The first thing that happens is--and this is the clonal selection theory. The first thing that happens is the antigen enters. AG is short for antigen. The antigen enters. Remember what's an antigen? An antigen--you know what it stands for really? I'm getting ahead of myself a little bit because you don't know what an antibody is. You might. An antibody is a chemical that are going to be produced by B cells. Actually, you should know that. Antigen stands for antibody generator. And antigen is going to be the foreign substance. So it's an antibody generator. It could be a toxin, it could be protein, it could be a glyco-protein on the cell wall of a bacterium. It could be anything. Something that's not you.
Now, remember, you have this diverse number of T cells and B cells. You have T cells and B cells floating through your blood, and massed in your lymph nodes that are literally made for one particular chemical. Specific. How cool is that? So we've got a diversity of specific cells that can recognize self versus non-self. But there's more. So here's what's going to happen. The toxin is going to trigger a reaction in the B and T cells. So here's the thing. Along comes a toxin, along comes the antigen, and it's got this particular shape. So let's say a B cell. And the B cell has a receptor on it and that antigen fits right in place right there. Maybe say something like this. Okay. You can't see it, but our wonderful Thinkwell artists have each of these with a different receptor end. But this particular B cell right here fits these little triangles, and therefore, that is going to trigger something in this B cell. Now, could this be a T cell, too? Absolutely, so I should say in this B or T cell. And here's what's going to happen. So the B and T cell have the antigen receptor and about 100,000 of these, certainly not four, and what that does is it's going to be activated now, and so now you get into what is going to be called an activated lymphocyte. They get activated. So literally here's what's going to happen. You are going to take this lymphocyte, which is a mature, but undifferentiated cell, and this lymphocyte is going to get activated and you are going to get what are called "effector cells." When we use the word "effector" in biology we mean it makes a response. And here's what's going to happen. If it's a B cell, it's going to become a plasma cell. And if it is a T cell it's going to be called--you know, T cells--I've got to get vague with you--we'll call it an effector T. In other words, it's going to effect a response. And what might that be? Well, I'll show you.
Then what happens is this triggers cloning, and look at this. The first thing that happens is you get a group of these plasma cells and they clone. And they clone to produce antibodies. So plasma cells are going to produce antibodies, but guess what else is going to happen? They're also going to clone a group of cells that aren't going to be necessarily antibody producers actively, but they are going to be called memory cells. Number four, memory. These guys, thousands of them, will stay in your blood stream the rest of your life. So once you've been exposed to this antigen, you've activated a group of warriors and a group of rememberers. So we have the scribes over here remembering that we never want this to happen again, and we've got these soldiers over here fighting the battle, and it's the same thing with T cells.
The T cells, instead of a B cell or plasma cell, might turn into something called the "cytotoxic T cell," which attacks cells, and will turn into a T memory cell. So you get workers and you get rememberers. You just got cell memory. Let's take a look at what I mean. Here it is, the whole summary of the whole story right here, guys. Look. Somebody coughs on you. What starts to happen? Well, you get sick. You get strep throat. You're miserable. We call this the primary immune response. What's happening? You're making antibodies. Okay, somebody coughs on you here, you start making antibodies here, you're starting to feel pretty miserable. You're sick, you're sick, you're sick, you're sick. Your antibodies get way high. You get better, you get better, you get better, you get better. About two or three weeks, three to five days; it depends on the disease.
But then what happens? We could say ten years later if you want, rather than ten days later, you get exposed. So right here we're going to say--we'll just call this "the second exposure." Your little nephew comes up and says, "Uncle Harry, I don't feel good," coughs all over you, germs you real good. What's going to happen now? Look at this. Look how quickly you make those antibodies. This baby just goes "vroom," within like two days. You never get sick. You are immune because you have the memory cells. But, of course, your nephew might have had strep throat and a cold, and he germed you with a cold, too. And you get the cold, but you never get the strep throat. So you get memory after memory after memory. So one great thing about age--you get exposed to all these germs--you never get sick anymore. You can hang around all these young people who keep getting sick and you don't get sick.
So there it is. It's the whole idea of the clonal selection and memory in immunology. We'll take a look at more details later.
Animal Systems and Homeostasis
The Immune System: An Introduction
Immunity: Clonal Selection Theory Page [1 of 2]

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