Clonal Selection - Online Tutor, Practice Problems & Exam Prep

The clonal selection theory explains how specific B and T cells are activated in response to an antigen. Each B and T cell has unique BCRs or TCRs that bind to specific antigens. Upon infection, only the B and T cells that recognize the antigen proliferate, creating clones that can effectively combat the pathogen. Activated B cells 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 adaptive immunity against specific antigens.

Clonal Selection

In this video, we're going to begin our lesson on a process known as clonal selection. 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 allow 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. 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.

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. 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. What this means is that all of the other B and T cells that do not bind the very specific antigen that we are infected with, are not going to be selected to proliferate. 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.

We can get a better understanding of the clonal selection theory by taking a look at our example below of the clonal selection theory where we apply it to B cells. But again, it also applies to T cells as well. 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. Notice in this image we have 3 different layers: the top layer, the middle layer, and the bottom layer. At the top in this first layer, what we're showing you is the mixed B cell population. 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.

Notice that in this image, the antigen that is present is this red antigen. 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. Proliferation just means to multiply and create identical clones. Notice that it's only B cell number 2 that has the BCRs to bind and respond to this very specific antigen. So, it's B cell number 2, not B cell number 1, nor B cell number 3, that is going to be selected to proliferate. Notice that this B cell becomes activated and it begins proliferating or making clones, dividing. Now we have an army of clones that are capable of responding to this very specific antigen that we are infected with.

Then, after B cell activation and proliferation, of course, what we have next is the differentiation of the activated B cells. That means 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. Notice here that this activated B cell is differentiating into these plasma cells which will again secrete antibodies, producing antibodies. These antibodies will be specific to the very specific antigen that we are infected with, helping to eliminate that particular antigen. Again, notice that some of the B cells, instead of differentiating into plasma cells, will differentiate into memory B cells, and they'll be able to respond even faster upon a secondary infection.

This 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. These other B cells that are here, they will not proliferate unless their very specific antigen is present. Again, this concludes our brief lesson on the 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 immunology?

Clonal selection is a fundamental concept in immunology that explains how specific B and T cells are activated in response to an antigen. Each B and T cell has unique receptors (BCRs or TCRs) that bind to specific antigens. When an antigen enters the body, only the B and T cells that recognize this antigen are selected to proliferate. These selected cells then create clones of themselves, which can effectively combat the pathogen. This process ensures a targeted immune response, enhancing the body's ability to fight specific infections.

How does clonal selection contribute to adaptive immunity?

Clonal selection contributes to adaptive immunity by ensuring that only the B and T cells specific to an invading antigen are activated and proliferate. This selective process creates a large number of identical cells (clones) that can effectively target and eliminate the pathogen. Additionally, some of these activated B cells differentiate into memory B cells, which provide a faster and more robust response if the same antigen is encountered again in the future. This targeted and efficient response is a hallmark of adaptive immunity.

What role do memory B cells play in clonal selection?

Memory B cells play a crucial role in clonal selection by providing long-term immunity. After the initial infection, some of the activated B cells differentiate into memory B cells instead of plasma cells. These memory B cells persist in the body and can quickly respond to subsequent exposures to the same antigen. Upon re-exposure, they rapidly proliferate and differentiate into plasma cells, producing antibodies more quickly and effectively than during the first encounter. This results in a faster and more efficient immune response.

What is the difference between BCRs and TCRs in clonal selection?

BCRs (B cell receptors) and TCRs (T cell receptors) are both crucial for clonal selection, but they have different roles. BCRs are found on the surface of B cells and are responsible for recognizing and binding to specific antigens directly. Upon binding, B cells are activated, proliferate, and differentiate into plasma cells and memory B cells. TCRs, on the other hand, are found on T cells and recognize antigens that are presented by other cells via major histocompatibility complex (MHC) molecules. This recognition leads to the activation, proliferation, and differentiation of T cells, which then help in various immune responses.

How does clonal selection ensure a targeted immune response?

Clonal selection ensures a targeted immune response by selectively activating only those B and T cells that have receptors specific to the invading antigen. When an antigen enters the body, it binds to the receptors on a small subset of B and T cells. These specific cells are then activated to proliferate and create clones of themselves. This results in a large population of cells that are all capable of recognizing and attacking the same antigen, thereby providing a highly targeted and efficient immune response against the specific pathogen.