Previously, we said that B cells can encounter antigens that are just free-floating in the body, but T cells need to be presented antigens. T cells are actually going to be classified as either CD4+ or CD8+ based on whether they have a CD4 or CD8 protein. Now, these CD4+ T cells will interact with epitopes bound to MHC class 2 proteins. You can see this happening here. Those MHC class 2 proteins, remember, are going to be found on antigen-presenting cells like dendritic cells. When these CD8+ T cells are presented with epitopes that they can interact with, they become activated. These activated CD4+ T cells will undergo clonal expansion and form helper T cells.
The CD8+ T cells will interact with epitopes on MHC class 1 proteins, which are going to be found on most cells. So, you know, these aren't going to be those specific antigen-presenting cells like the dendritic cells and the B cells and the macrophages. Instead, these are going to be found on just like your average regular Joe cells. If they can bind to those epitopes, it's going to activate those CD8+ T cells, and those will undergo clonal expansion to form cytotoxic T cells. So here behind me, you can see the cytotoxic T cells being formed from antigen presentation with an MHC class 1 protein. And over here, you can see helper T cells being formed from an interaction with an MHC class 2 protein. Remember, these are going to be the antigen-presenting cells, like dendritic cells. Now, basically, what these cells are asking is, "Show me what you've got."
Let's talk about what these types of cells do. Cytotoxic T cells, which are sometimes abbreviated as TC, are effector T cells, and their job is to kill pathogen-infected cells. This is basically a defensive measure against the pathogen. So, if this cell, let's say, has been infected and it is presenting the antigen from that pathogen, the cytotoxic or killer T cell will come over and induce cell death. It will induce the cell to die.
With that, let's flip the page and talk about what helper T cells do.
