In this video, we're going to begin our lesson on T-dependent antigens and T-independent antigens. And so it turns out that the mechanism of B cell activation actually depends on the type of antigen that that B cell encounters. And so when it comes to B cell activation, really there are two types of antigens that we need to introduce. And so the first type of antigen is going to be called T-dependent antigens, and the second type of antigens are called T-independent antigens. Now the T-dependent antigens, as their name implies, these are going to be antigens that depend on, or in other words, require helper T cells or TH cells in order for the activation of naive B cells. Now on the other hand, the T-independent antigens, as their name implies, are going to be antigens that are capable of activating naive B cells independent of or in other words without helper T cells or TH cells. And so, really, what we're saying here is that T-dependent antigens depend on helper T cells in order for a B cell to become activated. And, again, T-independent antigens do not require helper T cells. They are able to activate B cells, independent of or without helper T cells. Now it turns out that most of the antigens are going to be T-dependent antigens and the activation of B cells via T-dependent antigens, as well as the activation of B cells via T-independent antigens. And so I'll see you all in our next video.
T Dependent & T Independent Antigens - Online Tutor, Practice Problems & Exam Prep
T Dependent & T Independent Antigens
Video transcript
B Cell Activation by T Dependent Antigens
Video transcript
In this video, we're going to begin our lesson on B cell activation by T-dependent antigens. And so first, we need to recall from some of our previous lesson videos that naive B cells or inactive B cells can be activated by helper T cells or TH cells. And so, activation of a naive B cell via T-dependent antigens actually occurs via a series of 5 steps that you can see in our image down below and are numbered 1 through 5. And so, over here on the far left of our image, we have the very first step of T-dependent antigen B cell activation. And so, notice that we're showing you an image over here of a naive B cell or an inactive B cell. And of course, the B cell on its surface is going to have thousands of identical B cell receptors or BCRs. And so, here what we're saying in the first step is that the B cell receptor or the BCR is going to bind the free antigen or the free-floating antigen. And so, you can see the antigen here is in red, and you can see that our BCR here is binding to that free antigen.
Now, once the antigen has been bound by the BCR, in the second step, the B cell is then going to process the antigen. It's going to process the antigen into smaller fragments. And so, in order to do that, the antigen needs to be internalized inside of the B cell, and then it will be fragmented and broken down into smaller pieces, smaller fragments. Then in the 3rd step, the B cell is then going to present those antigen fragments on its surface, on MHC class 2 molecules, which we call are on the surface of the cell. And so, notice down below we're indicating that the MHC class 2 molecule is going to look like this in our image. And notice that these MHCs are presenting the smaller fragments of the antigen that was internalized and processed. And recall from our previous lesson videos that these MHCs or major histocompatibility complexes, specifically class 2, when antigens are presented on MHC class 2, it's the helper T cells that can recognize those presented antigens.
And so, in step number 4, what you'll notice is that the helper T cell, or the TH cell, is going to recognize those antigens that are presented on the MHC class twos. And so, the helper T cell will go on to activate the naive B cell. And so, notice over here we have our helper T cell, in our image, our TH cell. And of course, the helper T cell has those CD4 molecules. And so, the T cell receptors of helper T cells will only detect presented antigens on MHC class 2 molecules. And so, notice the interaction here is the TCR, is recognizing the antigen presented on the MHC class 2. And so, that will allow the helper T cell to release a series of cytokines or communicating signals or molecules. And those cytokines released by the helper T cell will be able to affect the B cell in such a way that it will activate that naive B cell. And of course, we know from our previous lesson videos that activated B cells will go on to differentiate into either plasma cells that secrete antibodies or differentiate into memory B cells, that will be important for a secondary future infection.
And so, notice down below we're showing you the plasma cells over here on the left secreting these antibodies, and the memory B cell over here on the right. And so, ultimately what we've seen is that T-dependent antigens depend on helper T cells in order to activate the B cells. And so, this here is going to be the key feature of T-dependent antigens, the fact that they depend on helper T cells. And again, this will be different when we talk about T-independent antigens as we move forward in our course. But for now, this here concludes our brief lesson on B-cell activation by T-dependent antigens, and we'll be able to get some practice moving forward. So, I'll see you all in our next video.
Which of the following is a step required for activation of a B cell by a T-dependent antigen?
T-dependent antigens can stimulate B cells to become activated but require _________ assistance.
B Cell Activation by T Independent Antigens
Video transcript
In this video, we're going to talk more about B cell activation by T-independent antigens. First, we need to recall from some of our previous lesson videos that unlike T-dependent antigens, T-independent antigens can actually activate naive B cells independent of helper T cells, or in other words, without helper T cells. These T-independent antigens are typically long polysaccharides with multiple closely spaced identical repeating subunits. Additionally, these T-independent antigens typically do not initiate an immune response in very young children, which makes very young children and kids more susceptible to some pathogens that have these T-independent antigens. Fortunately, these T-independent antigens are not as common as T-dependent antigens.
If we take a look at our image below, we can get a better understanding of these T-independent antigens. On the left-hand side of our image, we have our naive B cell in blue, which of course is going to have its B cell receptors (BCRs) on its surface. Notice that here in red, this long structure that you see represents our T-independent antigen. These T-independent antigens are going to be long polysaccharides with multiple closely spaced identical repeating subunits. This allows these T-independent antigens to activate the naive B cell without a helper T cell. Notice that there is no helper T cell anywhere within this image. Therefore, B cell activation can still occur with these T-independent antigens. Once the naive B cell becomes activated, it can proliferate or multiply and differentiate either into plasma cells that secrete antibodies or memory B cells that are important for protecting against a future infection.
Really, the key here is that T-dependent antigens require helper T cells, whereas T-independent antigens do not require helper T cells to activate a B cell. This concludes our brief lesson on B cell activation by T-independent antigens, and we'll be able to get some practice applying these concepts moving forward. I'll see you all in our next video.
T-independent antigens:
Lipopolysaccharide (LPS) is an endotoxin found on the surface of gram-negative bacteria. LPS can cause the body to enter septic shock and result in multi-system organ failure. Because of its severe effects, it is important that the immune cells react quickly to LPS. LPS is an antigen that can directly trigger antibody secretion once it comes in contact with a B cell. LPS is what kind of antigen?
