Introduction to B Lymphocytes: Videos & Practice Problems

B lymphocytes, or B cells, are crucial for adaptive humoral immunity, targeting extracellular pathogens through antibodies. They develop in the bone marrow and possess B cell receptors (BCRs) that bind free antigens. Upon activation by helper T cells, naive B cells can proliferate and differentiate into effector plasma cells, which produce antibodies, or memory B cells, which enhance future immune responses. Understanding B cell activation and differentiation is essential for grasping immune system functions and responses to infections.

Introduction to B Lymphocytes

In this video, we're going to begin our introduction to B lymphocytes or B cells. And so first, we need to recall from some of our previous lesson videos that B lymphocytes or B cells play an important role in adaptive humoral immunity. Recall that humoral immunity is the part of the adaptive immune system that is going to be targeting and destroying extracellular pathogens or pathogens that are on the outside of host cells. This humoral immunity will be able to destroy these extracellular pathogens by using B cells, as well as using what are known as antibodies. We'll get to talk a lot more about antibodies as we move forward in our course.

Now also recall from some of our previous lesson videos that dendritic cells, which are antigen-presenting cells or an APC, have both MHC class 1 and MHC class 2 molecules. That means that they're capable of activating T cells, both types of T cells, the cytotoxic and the helper T cell. When they activate the helper T cell, the helper T cell will then go on to be able to activate the B cell, and then the B cell will be able to carry out its immune functions.

If we take a look at our image down below, you'll notice it's an image that we've seen before in our previous lesson videos. Once again, we have our primary lymphoid organs at the top which include the thymus and the bone marrow. The thymus is where the T cells fully develop and the bone marrow is where the B cells fully develop. Notice that because we've briefly discussed the T cells already, the T cell area over here is pretty much all grayed out because we've already talked about it briefly in our previous lesson videos. In this video, we're going to be focusing on the B cells.

These fully matured naive B cells will migrate from the primary lymphoid organs to the secondary lymphoid organs, and so here we have a naive or inactive B cell. This naive B cell has not yet encountered an antigen, but when it does encounter its antigen, it will be able to internalize that antigen, process that antigen, and then display that antigen on its MHC class 2 molecules. When it's displaying those MHC class 2 molecules, those antigens on those MHC class 2 molecules, the effector helper T cell can recognize those antigens on the MHC class 2. The effector helper T cell can then release cytokines that can ultimately activate the B cell. The activated B cell, as we've discussed before in some of our previous lesson videos, is going to be able to proliferate or divide to create clones as well as differentiate into either memory B cells as you see here, and the memory B cells will be important for a secondary infection, or the activated B cells could differentiate into what are known as plasma cells, and plasma cells are really the effector B cell.

These plasma cells are going to be able to release and secrete antibodies, and these antibodies can carry out many different types of immune functions. As we move forward in our course, we're going to talk a lot more details about this activation process of the B cells, the differentiation of the memory and the plasma cells, and then also the antibodies that these plasma cells can produce. This here concludes our brief introduction to these B lymphocytes or B cells, and again, we'll be able to learn more as we move forward. So, I'll see you all in our next video.

Match the correct form of adaptive immunity to the scenarios below.
A. Cell-Mediated Immunity.                                B. Humoral Immunity
_____1. A macrophage acting as an APC is activated by a CD4 effector cell.
_____2. A naive B cell is activated after binding an antigen and differentiates into a plasma cell.  
_____3. Responds to exogenous antigens.  
_____4. Responds to endogenous antigens.
_____5. A liver cell infected with a virus undergoes apoptosis when signaled by a CD8 effector cell.  

Okay, everyone. Match the correct form of adaptive immunity to the scenarios below. We have cell-mediated immunity, and then we have humoral immunity. So, cell-mediated immunity involves all the functions of T cells and what they do. Humoral immunity encompasses all the functions of B cells and what they do. The only time these types of immunity overlap is when T cells activate B cells, and then these two types of immunity kind of overlap. But otherwise, we should be able to categorize scenarios into one type of immunity or another. So let's get started. Number 1 says a macrophage acting as an APC is activated by a CD4 effector cell. Remember, this is about antigen presentation. This is a professional APC and it is activated by a CD4 cell, which is a helper T cell. Since this involves a T cell, this is going to be cell-mediated immunity. Number 2, a naive B cell is activated after binding an antigen and differentiates into a plasma cell. We'll learn about this soon, but plasma cells are the cells that create antibodies. So, this is part of humoral immunity because it involves B cells in the production of antibodies. Number 3 responds to exogenous antigens. Remember, exogenous means outside the cell. So, these are B cells. This is going to be a form of humoral immunity. Number 4, for example, if a cell is infected by a virus and then presents those antigens on its surface, those are endogenous antigens. These are going to be acknowledged and responded to by T cells. So this is cell-mediated immunity. Finally, number 5, a liver cell infected with a virus undergoes apoptosis when signaled by a CD8 effector T cell. Remember that the CD8 effector cell is also known as the cytotoxic T cell and this is a T cell. So, this is going to be cell-mediated immunity. So, that is how we're going to organize our scenarios into either cell-mediated immunity or humoral immunity. Okay, everyone. Let's move on to our next video.

B cell Receptors

In this video, we're going to briefly discuss B cell receptors. First, we need to recall from some of our previous lesson videos that B cells develop in the bone marrow. These B cells have thousands of identical B cell receptors embedded in their membranes. These B cells are also associated with what are known as antibodies. That's because these B cells have the ability to differentiate into what are known as plasma cells that secrete many antibodies. We'll get to talk a lot more about plasma cells and antibodies as we move forward in our course. But going back to these B cell receptors, these B cell receptors are commonly abbreviated as BCRs. Again, each B cell is going to have thousands of identical BCRs embedded in their membrane.

Now, these B cell receptors or BCRs, are receptors that allow B cells to recognize and attack extracellular pathogens. One of the main differences between B cell receptors, or BCRs, and T cell receptors, or TCRs, is that, recall, TCRs do not bind to free antigens. TCRs can only bind to presented antigens on MHCs. However, B cell receptors can actually bind to free-floating antigens. The free-floating antigens are able to bind to the BCRs. When these free-floating antigens bind to the BCRs, they can then be internalized and processed, and then those free-floating antigens can be presented by the cell for activation by helper T cells, presenting those free-floating antigens on their MHC class 2 molecules.

Some important things to note about these BCRs is that BCRs are practically membrane-embedded antibodies. The structures of antibodies are going to resemble the structure of BCRs and vice versa, and they have very similar overall structures. We'll get to talk about the structure of BCRs and antibodies in more detail as we move forward in our course.

Notice on the left-hand side over here, we're showing you a B cell. This B cell can have, again, thousands of identical BCRs embedded in their membrane, these are the B cell receptors. Notice that these BCRs take on a shape that resembles that of the antibodies they eventually produce. The BCRs are capable of binding to free antigens, antigens that are not being presented. If we zoom into this BCR over here, what you’ll notice is that the BCR has a Y-shaped structure you see right here. This is our B cell receptor, and this yellow region represents the antigen-binding site of the B cell receptor. This is the region of the BCR that binds the antigen. Notice that there are two antigen-binding sites allowing for the BCR to bind to two antigens at once. We're showing you a free antigen and notice that it's binding to this specific epitope of the free antigen.

This concludes our brief introduction to B cell receptors (BCRs) and their ability to bind to free antigens. We’ll be able to get some practice applying these concepts and learn more as we move forward in our course. I’ll see you all in our next video.

What is the difference between a TCR and a BCR?

• A. TCRs must be presented an antigen on an MHC molecule from an APC.
• B. BCRs must be presented an antigen on an MHC molecule from an APC.
• C. TCRs mimic the structure of antibodies and are essentially the same.
• D. BCRs are composed of amino acid chains & TCRs are composed of various carbohydrates.
• E. TCRs are composed of amino acid chains & BCRs are composed of various carbohydrates.

Answer: (A)

A naive B cell is activated when:

• A. The B cell's BCR binds to an endogenous antigen presented by an APC.
• B. The B cell is told to differentiate into a plasma cell or memory B cell by a CD4 effector cell.
• C. The B cell's BCR binds to a 'free' antigen that is not bound to an APC.
• D. When A and B occur.
• E. When B and C occur.

Answer: (E)

Naive B cells Become Effector (Plasma) Cells & Memory Cells

In this video, we're going to continue our introduction to B lymphocytes by talking about the ability for naive B cells to become effector plasma cells and memory B cells. First, we need to know that before a B cell encounters a free antigen, that B cell exists in an inactive form that we refer to as a naive B cell. Once again, a naive B cell is really just an inactive B cell that has not yet encountered its free antigen. Upon encountering its free antigen and then presenting that free antigen on an MHC class 2 molecule, the naive B cells are going to be bound and activated by helper T cells. Recall from our previous lesson videos that helper T cells can help to activate B cells. These activated B cells are capable of proliferating or multiplying to create identical clones of itself, as well as differentiating or changing its phenotype to become either an effector plasma cell or a memory B cell.

The effector plasma B cells are going to be short-lived cells that make antibodies. They can make thousands of antibodies per second. So, they're making lots and lots of antibodies. These antibodies, as we'll learn moving forward in our course, are capable of immediately responding to the first infection. They are able to respond to the infection in many different ways. The memory B cells, on the other hand, are long-lived cells and do not respond immediately to the first infection. Instead, these memory B cells, that are long-lived cells, somewhat remember the antigen and their future infection, and help eliminate that future infection even faster than the primary infection was eliminated.

If we take a look at our image down below, we can get a better understanding of the ability for the naive B cell to differentiate into either an effector plasma cell or a memory B cell. Notice over here on the left-hand side what we have is a naive B cell, an inactive B cell. When this naive B cell encounters a free antigen, as you see right here, a free-floating antigen that's not being presented, this free antigen can be internalized into the B cell, processed, and then presented on MHC class 2 molecules. The MHC class 2 molecules are going to allow helper T cells to activate the naive B cell. Activation of the naive B cell will allow the B cell to differentiate either into a memory B cell as we see right here, or it would allow the B cell to differentiate into a plasma cell. The plasma cell would be the effector cell, again capable of releasing and producing thousands of antibodies per second, releasing those antibodies into the environment, and those antibodies that are released into the environment can then carry out several different immune responses, which we'll get to talk about as we move forward in our course.

Recall that on these B cells, they have B cell receptors or BCRs. The BCRs that a B cell has are pretty much membrane-embedded versions of the antibodies that that B cell ends up producing after it differentiates into a plasma cell. This concludes our brief lesson on how naive B cells can become effector plasma cells and memory cells. We'll be able to get some practice applying these concepts and learn more as we move forward in our course. So, I'll see you all in our next video.

Antibodies are made by:

• A. Red blood cells.
• B. Plasma cells.
• C. Dendritic cells.
• D. Helper T cells.

Answer: (B)

In which of the following sites in the body can B cells be found?

• A. Lymph nodes.
• B. Spleen.
• C. Red bone marrow.
• D. Intestinal wall.
• E. All of the above.

Answer: (E)

The antibody-secreting progeny of an activated B cell are called:

• A. Antibodies.
• B. Sensitized T cells.
• C. Activated macrophages.
• D. Plasma cells.

Answer: (D)

Which of the following are properties of B cells?

• A. Naive B cells, like naive T cells, can differentiate into effector and memory B cells once activated.
• B. Effector B cells when activated secrete antibodies which fight a novel infection.
• C. Memory B cells when activated secrete antibodies which fight subsequent infections more effectively.
• D. To become activated, a naive B cell must present an immunogenic antigen to an effector CD4 cell.
• E. All are properties of B cells.

Answer: (E)

Here’s what students ask on this topic:

What role do B lymphocytes play in adaptive humoral immunity?

B lymphocytes, or B cells, are essential for adaptive humoral immunity, which targets extracellular pathogens. These pathogens are located outside host cells. B cells achieve this by producing antibodies that can neutralize or mark pathogens for destruction. Upon encountering an antigen, a naive B cell internalizes and processes it, presenting it on MHC class II molecules. Helper T cells recognize these antigens and activate the B cells through cytokine release. Activated B cells then proliferate and differentiate into plasma cells, which secrete antibodies, or memory B cells, which provide a faster response upon future infections.

How do B cell receptors (BCRs) function in the immune response?

B cell receptors (BCRs) are membrane-bound proteins on B cells that recognize and bind to free antigens. Each B cell has thousands of identical BCRs. When a BCR binds to an antigen, the antigen is internalized, processed, and presented on MHC class II molecules. This presentation allows helper T cells to activate the B cell. BCRs are structurally similar to antibodies, and they can bind to two antigens simultaneously due to their Y-shaped structure. This binding is crucial for the activation and subsequent immune response of B cells.

What is the difference between naive B cells, effector plasma cells, and memory B cells?

Naive B cells are inactive B cells that have not yet encountered an antigen. Upon encountering an antigen and receiving activation signals from helper T cells, naive B cells proliferate and differentiate. Effector plasma cells are one type of differentiated B cell that produces and secretes large quantities of antibodies to combat the current infection. These cells are short-lived. Memory B cells are another type of differentiated B cell that remains in the body for a long time, providing a rapid and robust response if the same antigen is encountered again in the future.

How do helper T cells activate B cells?

Helper T cells activate B cells through a process involving antigen presentation and cytokine release. When a B cell encounters an antigen, it internalizes and processes it, presenting the antigen on MHC class II molecules. Helper T cells recognize these antigens via their T cell receptors (TCRs) and bind to the MHC class II-antigen complex. This interaction, along with the release of cytokines by the helper T cells, activates the B cell. The activated B cell then proliferates and differentiates into plasma cells and memory B cells, enhancing the immune response.

What are the primary and secondary lymphoid organs involved in B cell development?

The primary lymphoid organ involved in B cell development is the bone marrow. This is where B cells fully mature. Once matured, naive B cells migrate to secondary lymphoid organs, such as the spleen, lymph nodes, and mucosal-associated lymphoid tissues (MALT). In these secondary lymphoid organs, B cells encounter antigens and undergo activation, proliferation, and differentiation into plasma cells and memory B cells, which are crucial for the adaptive immune response.