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 t h cells. And so the 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 third 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 t h 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 on it. It's also known as a CD4 cell, and it has t cell receptors that only detect presented antigens.
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.