Antibodies - Online Tutor, Practice Problems & Exam Prep

Antibodies, or immunoglobulins, are Y-shaped proteins produced by plasma cells that bind specifically to antigens, triggering an immune response. Each antibody consists of four polypeptide chains: two identical light chains and two identical heavy chains, linked by disulfide bonds. The variable region at the tips of the Y contains the antigen-binding site, while the constant region is recognized by immune cells. Antibody diversity arises from gene rearrangements, splicing, and mutations, allowing the immune system to produce an estimated 1 quintillion different antibodies, far exceeding the number of human genes.

Antibodies

In this video, we're going to begin our lesson on antibodies. Antibodies are defined as Y-shaped proteins that are actually produced by plasma cells, and they have the ability to bind very specifically to antigens and generate an immune response. Now, antibodies are also sometimes called immunoglobulins and abbreviated as Ig. And although there are five different classes of antibodies that we'll talk more about moving forward in our course, typically antibodies have the same general structure. In our next lesson video, we're going to talk more details about the structure of the antibody. I'll see you all in that video.

Antibody Structure

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.

_________ is another name for antibodies.

• A. Epitope.
• B. Immune protein.
• C. Antigen.
• D. Immunoglobulin.

An antibody’s variable region:

1.   Varies in amino acid sequence to allow different antibodies to bind different antigens.

2.   Is located in the hinge and stem regions of an antibody.

3.   Is a portion of the light chain of an antibody.

4.   Is a portion of the heavy chain of an antibody.

• A. 1 & 2.
• B. 1 & 3.
• C. 3 & 4.
• D. 1, 2, & 3.
• E. 1, 3, & 4.
• F. 1, 2, 3, & 4.

Antibody Diversity

In this video, we're going to talk about antibody diversity. It turns out that our immune systems have the potential to produce an enormous amount of different antibodies, perhaps greater than 1018 or 1 quintillion different antibodies. That's more antibodies than the estimated amount of individual grains of sand on our entire planet. That's a lot of antibodies that our immune system has the potential to produce. In fact, there are so many potential possibilities for antibodies that they all cannot be produced in one single lifetime. So, here we have a question and it's asking how in the world is it possible that antibody diversity can be so large if humans only have 25,000 genes, which is a much smaller number than 1 quintillion. The answer to this question is actually right here. Antibody diversity results from a significant amount of gene rearrangements, splicing, and mutations. Notice down below on the left-hand side, we have DNA being shown. This DNA is coding for an antibody. You can see that the different regions of the DNA are color-coded to show you what part of the antibody they express. Notice that the original DNA up here has many different combinations for these regions of the antibody. However, through splicing and rearrangements, we can get different smaller combinations, combining different features and even mutations in the DNA can create lots and lots of diversity. Through transcription of the DNA into RNA, you can see that even the mutation will carry over here. Then, through translation, what we get is the antibody being produced and even a single slight mutation like this one right here can result in a different antibody being produced. We get a diverse antibody just through all of these gene rearrangements, splicing, and mutations. This concludes our introduction to antibody diversity and in our next video, we'll be able to talk about monoclonal and polyclonal antibodies, so I'll see you guys there.

Genetic recombination frequently occurs in the body’s B cell population. Why is this advantageous to the immune system?

• A. More genetic diversity in antibody genes creates more diversity in antibodies.
• B. Having the ability to produce more diverse antibodies allows B cells to respond to a larger number of pathogens.
• C. More genetic diversity allows CD8 effector cells to be able to recognize and kill more endogenous pathogens.
• D. A and B.
• E. B and C.
• F. All of the above.