Antibodies: Study with Video Lessons, Practice Problems & Examples

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 regions at the tips of the Y contain the antigen-binding sites, while the constant regions interact with immune cells. Antibody diversity arises from gene rearrangements, splicing, and mutations, allowing the immune system to produce over 1 quintillion different antibodies, far exceeding the 25,000 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. Antibodies are also sometimes called immunoglobulins and abbreviated as IG. 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 details about antibody structure. 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. The light chains are actually much lighter than the heavy chains, which are much much heavier and larger. These 4 chains are covalently linked together via disulfide bonds. If we take a look down below at our image, notice we have our Y-shaped antibody. 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. The heavier chains, highlighted in green, are much larger and therefore much heavier than the lighter chains, which are much smaller and lighter in mass. Also notice that these four chains are covalently linked together via these disulfide bonds that exist between the R groups of cysteine residues.

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. 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. 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. It also contains the N-terminal end or the free amino groups of each of the 4 polypeptide chains just like what we said above. The V domain contains the antigen binding site. This is where the antibody is going to bind to the antigen at these two potential positions indicated by the arrows. The reason it's called the V or variable domain is because this region will actually vary between different antibodies. The C domain, on the other hand, being the constant domain, is not going to vary. It's going to remain constant even between different antibodies. The C domain is, of course, going to be the rest of the antibody, located at the hinge and the stem of the Y. So if we look down below, the C domain is going to be the rest of the antibody here. The C domain is important because it's actually recognized by immune system cells. 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. The antibody can act as an intermediate between the immune system cell and the antigen.

What you'll notice in our image here, we have the V domain, with the V's on them and whether or not they're light or heavy chains is indicated by the L and the H. Both the heavy chain and the light chain have a constant domain as well. 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. Here in a dotted red line, what we have is an imaginary line if we were to break our antibody right at the hinge. This would leave us with the top portion here, which we would refer to as the FAB region and this has the fragment that has the antigen binding sites. We have the antigen binding site in this region. 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. This would be the FC region, this bracket right here. 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. And so, it turns out that our immune systems have the potential to produce an enormous amount of different antibodies, perhaps greater than \(10^{18}\) 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. And so the answer to this question is actually right here. And so antibody diversity actually results from a significant amount of gene rearrangements. Splicing, and mutations. And so, notice down below over here on the left-hand side, we have DNA being shown. This DNA is coding for an antibody. And you can see that the different regions of the DNA are color-coded to show you what part of the antibody they express. And so, notice that the original DNA up here has many different combinations for these different 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. And so, through the transcription of the DNA into RNA, you can see that even the mutation will carry over here. And 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. And so we get a diverse antibody just through all of these gene rearrangements, splicing, and mutations. And so 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.

Here’s what students ask on this topic:

What are antibodies and how do they function in the immune system?

Antibodies, also known as immunoglobulins (Ig), are Y-shaped proteins produced by plasma cells. They play a crucial role in the immune system by specifically binding to antigens, which are foreign substances like bacteria and viruses. This binding triggers an immune response to neutralize or destroy the antigen. Each antibody consists of four polypeptide chains: two identical light chains and two identical heavy chains, linked by disulfide bonds. The tips of the Y-shaped structure contain variable regions that bind to antigens, while the constant regions interact with immune cells, facilitating the immune response.

What is the structure of an antibody?

An antibody consists of four polypeptide chains: two identical light chains and two identical heavy chains, linked by disulfide bonds. The structure is Y-shaped, with the variable regions (V domains) located at the tips of the Y, which are responsible for antigen binding. The constant regions (C domains) form the rest of the antibody, including the hinge and stem of the Y. The V domains vary between different antibodies, allowing for diverse antigen binding, while the C domains remain constant and are recognized by immune cells.

How is antibody diversity generated?

Antibody diversity is generated through gene rearrangements, splicing, and mutations. Although humans have only about 25,000 genes, the immune system can produce over 1 quintillion different antibodies. This diversity arises from the recombination of different gene segments during B cell development, alternative splicing of RNA transcripts, and somatic hypermutation, which introduces mutations in the antibody genes. These processes create a vast array of antibodies, each with unique antigen-binding sites, enabling the immune system to recognize and respond to a wide variety of antigens.

What are the different classes of antibodies?

There are five main classes of antibodies, also known as immunoglobulins (Ig): IgG, IgA, IgM, IgE, and IgD. Each class has a distinct structure and function. IgG is the most abundant and provides long-term immunity. IgA is found in mucous membranes and body secretions. IgM is the first antibody produced in response to an infection. IgE is involved in allergic reactions and defense against parasitic infections. IgD is present on the surface of B cells and plays a role in initiating immune responses. These classes allow the immune system to effectively respond to different types of pathogens.

What is the role of the variable and constant regions in an antibody?

The variable regions (V domains) of an antibody are located at the tips of the Y-shaped structure and are responsible for binding to specific antigens. These regions vary between different antibodies, allowing for a diverse range of antigen recognition. The constant regions (C domains) form the rest of the antibody, including the hinge and stem of the Y. The C domains do not vary between different antibodies and are recognized by immune cells. This interaction facilitates the immune response, as the antibody acts as an intermediary between the antigen and the immune cells.