As I've said before, a single antigen can have many different epitopes in its structure. B cell receptors will bind to epitopes on complete antigens, but T cell receptors bind to epitopes that have been processed and presented. The key to this epitope binding and the specificity of the adaptive immune system lies in the genetics of the antigen receptors, and it has to do with what are called constant regions and variable regions. So, constant regions, as the name implies, remain constant, and those are parts of the light chain and heavy chain, and the alpha and beta chains depending on whether you're talking about B cell receptors or T cell receptors. That will be the same in each isotype of that receptor and here you can see those, highlighted in these darker regions. So, these are going to be the constant regions, or we'll just abbreviate it, C regions. Whereas these lighter colored regions, which I'll mark in red just to be extra clear, these are going to be our variable regions or V regions. So how does this all work? How do these variable regions vary from cell to cell? Well, the genes for antigen receptors have many V regions actually, and they also have other types of regions, and these regions will recombine in unique ways to produce unique structures. This is kind of the key to how these antibodies and antigen receptors get that specificity to an antigen.
For example, the light chain actually has about 40 variable regions, as well as what are called 5 joining segments, and between these alone, you can get over 2, or sorry, not over, exactly 200 possible combinations. So, a huge amount of variation. There are actually other regions involved that add more variation, but that's kind of beyond the level of understanding that we need. So, what happens is, as lymphocytes mature, there's genetic recombination of the various regions that will result in unique antigen receptors. And essentially, this genetic recombination is the crux of the specificity and flexibility of the adaptive immune system. And what's so amazing, getting back to that self, non-self recognition we talked about, is that if maturing B and T cells have antigen receptors that bind to self molecules, they are destroyed or deactivated. So, the adaptive immune system has a way of guarding against a T cell or a B cell accidentally developing a type of antigen receptor stuff. And here, you can see that recombination sort of modeled out. You know, I don't really want you to worry about the specifics. All you need to know is that there's a variety of regions that are essentially mixed and matched to create unique receptors.
So with that, let's flip the page.