So, from our last lesson video, we already know that antibodies are Y-shaped proteins. But in this video, we're going to talk more about antibody structure. And so antibodies actually consist of 4 polypeptide chains, 2 of which are identical light chains or L chains for short, and the other 2 are identical heavy chains or H chains for short. Now the light chains are actually much lighter than the heavy chains which are much much heavier and larger. Now these 4 chains are covalently linked together via disulfide bonds. And so if we take a look down below at our image, notice we have our Y-shaped antibody right here. And notice that our Y-shaped antibody has 4 polypeptide chains. It has this light chain that is identical to this light chain over here and then it has this heavy chain, right here that is identical to this heavy chain right here. And so you can see that the heavier chains highlighted in green are much larger and therefore much heavier than the lighter chains which are much smaller and much lighter in mass. And also notice that these four chains are covalently linked together via these disulfide bonds that exist between the R groups of cysteine residues.

Now, what's also important to note is that each of these light and heavy chains has a variable region also known as a V domain as well as a constant region also known as a C domain. Now the variable region or the V domain is going to be located at the tip of each of the prongs of the Y and it contains the N-terminal end of each of the polypeptide chains. So, if we take a look down below at our image, notice that the V or variable domain is highlighted with a green background right here at the tips of the prongs of the Y. And notice that it also contains the N-terminal end or the free amino groups of each of the 4 polypeptide chains just like what we said up above. Now, what's also important to note is that the V domain contains the antigen-binding site. So, this is where the antibody is going to bind to the antigen at these two potential positions indicated by the arrows. And so the reason it's called the V or variable domain is because this region right here will actually vary between different antibodies. Now, the C domain on the other hand, because it's the constant domain, it's not actually going to vary. It's gonna remain constant even between different antibodies. Now the C domain is, of course, going to be the rest of the antibody. So it's gonna be located at the hinge and the stem of the Y. So, if we take a look down below, of course, the C domain, is going to be the rest of the antibody here. And the C domain is important because it's actually recognized by immune system cells. And so essentially, what happens is the V domain will bind to the antigen at these positions and then an immune system cell can bind to the C domain. And so the antibody can act as an intermediate between the immune system cell and the antigen. Now, what you'll notice is in our image here, we have the V domain, here with the V's on them and then whether or not they're light or heavy chains is indicated by the L and the H. And so, you can see that both the heavy chain here and the light chain has a constant domain. And so, what's important to note is that we can further break up the structure of this antibody. If we imagine breaking the antibody at the hinge of the Y. And so here in a dotted red line, what we have is an imaginary line if we were to break our antibody right at the hinge. And what that would do is it would leave us with the top portion here, which we would refer to as the FAB region and this is because this has the fragment that has the antigen-binding sites. So you can see we have the antigen-binding site in this region. And then we're also left with the bottom half of the antibody down below, which would be the FC region or the fragment that contains the constant region. So this would be the FC region, this bracket right here. And so really this is the structure of a typical antibody and we'll be able to get some practice applying the concepts that we've learned as we move forward in our course. So I'll see you guys in our next video.