In this video, we're going to distinguish between free antigens and presented antigens that are found on what are known as APCs. First, it's important to note that B cells only respond to what are known as free antigens. As their name implies, free antigens are free-floating in the environment. This just means that these free antigens are antigens that are not being presented by another host cell. It's also important to note that T cells, on the other hand, are only going to respond to antigens that are presented. Or in other words, T cells only respond to presented antigens. Those presented antigens are going to be presented by antigen presenting cells. Now, antigen presenting cells are commonly abbreviated as APCs for short. These antigen presenting cells, or APCs, are really just immune cells that will process and present antigens in order for T cell activation to occur, activating the T cell so that the T cell can carry out an immune response. The APCs that will be important for you to know throughout this course are dendritic cells, macrophages, and B cells themselves can actually serve as antigen presenting cells. We'll get to talk more about these APCs as we move forward in our course as well. But for now, let's take a look at our image down below to get a better understanding of the difference between free antigens and presented antigens.
Notice on the left-hand side, what we're showing you is a B cell. We know that B cells have B cell receptors or BCRs. That's what these receptors are sticking out of the membrane of these B cells. Notice that B cells only respond to free antigens. once again, free antigens are free-floating in the environment. Notice this antigen right here is a free antigen because it is just floating around in the environment. When the free antigen binds to the BCR, that can potentially lead to the activation of the B cell and allow the B cell to carry out its immune functions.
On the right-hand side over here, notice that we're showing you presented antigens. Recall that T cells only respond to presented antigens, and those presented antigens will be presented by antigen presenting cells. Notice over here in blue, we have an antigen presenting cell; an APC. We're specifically showing you a dendritic cell here in this image. This antigen presenting cell can process an antigen and present that antigen on the surface using one of its molecules on the surface. This molecule right here is an antigen, but it is a presented antigen. It is being presented by an antigen presenting cell. The T cell over here will only be able to recognize antigens that are being presented by antigen presenting cells. Therefore, T cells don't respond to free antigens; they only respond to presented antigens. Again, the B cells will only respond to free antigens, not presented antigens.
This here concludes our brief lesson on the difference between free antigens and presented antigens on APCs. Once again, we'll be able to learn more about these concepts as we move forward in our course. I'll see you all in our next video.