Clonal Selection: Study with Video Lessons, Practice Problems & Examples

Clonal selection is a vital process in the adaptive immune system, where specific B and T cells are activated in response to a particular antigen. Each B cell has identical B cell receptors (BCRs) that recognize different antigens. Upon infection, only the B cell with the matching BCR proliferates, creating clones that can effectively combat the antigen. These clones differentiate into plasma cells, which secrete antibodies, or memory B cells for faster responses to future infections. This selective proliferation ensures a targeted immune response, enhancing overall immunity.

Clonal Selection

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.

Which of the following statements is FALSE?

• A. Each B cell has BCRs that bind to a single antigen.
• B. The cell type that secretes antibodies is called a plasma cell.
• C. A BCR allows naive B cells to detect an antigen.
• D. All naive B cells that are close in proximity to an antigen begin to differentiate.

Clonal selection:

• A. Implies that each individual lymphocyte produces a single antibody.
• B. Describes how the adaptive immune system can produce millions of different antibodies.
• C. Depends on an antibody recognizing a specific epitope.
• D. Is based on random naive B cells proliferating and differentiating.

The clonal selection theory states that:

• A. Self-reacting T cells are destroyed in the bone marrow.
• B. B cells will only proliferate during an infection if their BCRs successfully bind to the pathogen.
• C. Antibody structure changes as it encounters an antigen for higher specificity binding.
• D. Each T cell produces many different types of antibodies.

Which of the following is NOT an organ where clonal selection occurs?

• A. Spleen.
• B. Lymph nodes.
• C. Bone marrow.
• D. Mucosa associated lymphoid tissue.

All of the following are postulates of the clonal selection theory EXCEPT:

• A. Each B lymphocyte bears a single type of receptor with a unique specificity.
• B. BCR occupation is required for cell activation.
• C. Differentiated cells derived from an activated B lymphocyte bear receptors of similar specificity as the parent cell.
• D. B lymphocytes bearing receptors for self-molecules are destroyed at an early stage.
• E. All are postulates of the clonal selection theory.

Here’s what students ask on this topic:

What is clonal selection in the immune system?

Clonal selection is a fundamental process in the adaptive immune system where specific B and T cells are activated in response to a particular antigen. Each B cell has identical B cell receptors (BCRs) that recognize different antigens. Upon infection, only the B cell with the matching BCR proliferates, creating clones that can effectively combat the antigen. These clones differentiate into plasma cells, which secrete antibodies, or memory B cells for faster responses to future infections. This selective proliferation ensures a targeted immune response, enhancing overall immunity.

How does clonal selection theory explain the immune response to specific antigens?

Clonal selection theory explains that upon encountering a specific antigen, only the B and T cells with receptors (BCRs and TCRs) that match the antigen are selected to proliferate. These selected cells then multiply, creating identical clones that can effectively target the antigen. The clones differentiate into plasma cells, which produce antibodies, or memory cells, which provide a faster response if the antigen is encountered again. This process ensures that the immune system mounts a precise and efficient response to the specific antigen.

What roles do plasma cells and memory B cells play in clonal selection?

In clonal selection, plasma cells and memory B cells play crucial roles. Plasma cells are differentiated B cells that secrete large amounts of antibodies specific to the antigen, helping to neutralize and eliminate the pathogen. Memory B cells, on the other hand, are long-lived cells that remain in the body after the initial infection. They provide a rapid and robust response if the same antigen is encountered again, ensuring quicker and more effective immunity during subsequent infections.

Why is clonal selection important for adaptive immunity?

Clonal selection is vital for adaptive immunity because it ensures a highly specific and efficient immune response. By selecting only the B and T cells that recognize a particular antigen, the immune system can focus its resources on combating the specific pathogen. This targeted response not only helps in effectively eliminating the infection but also creates memory cells that provide long-term immunity. This specificity and memory are key features that distinguish adaptive immunity from the innate immune response.

How do B cell receptors (BCRs) and T cell receptors (TCRs) function in clonal selection?

B cell receptors (BCRs) and T cell receptors (TCRs) are crucial for clonal selection. BCRs are membrane-bound antibodies on B cells that bind to specific antigens. When a BCR binds its matching antigen, the B cell is activated to proliferate and differentiate into plasma cells and memory B cells. Similarly, TCRs on T cells recognize antigens presented by other cells. Upon binding to their specific antigen, T cells are activated to proliferate and differentiate into effector T cells and memory T cells. This receptor-antigen interaction is the basis for the selective activation and proliferation of specific immune cells.