Classes of Antibodies: Videos & Practice Problems

There are five classes of antibodies: IgG, IgA, IgM, IgE, and IgD, distinguished by their unique heavy chains. IgG is the most abundant, providing immunity to fetuses and newborns by crossing the placenta. IgA protects mucous membranes and is found in secretions like saliva and breast milk. IgM is the first antibody produced during infection, effective in blood. IgE is involved in allergic reactions and parasite defense, while IgD aids in B cell activation. Understanding these classes is crucial for grasping adaptive immunity and immune responses.

Classes of Antibodies

In this video, we're going to talk about the different antibody classes and so it turns out that there are actually 5 different classes of antibodies or in other words, 5 different classes of immunoglobulins. And so these five classes are based on differences in their heavy chains as we'll see down below in our table. And so the 5 classes of antibodies are IgG, IgA, IgM, IgE, and IgD. And if you look at these red letters here, notice that it spells GAMED, and so notice down below right here we have this antibody stick figure playing some video games, and based on his facial expression, I'd say he's pretty gamed out from playing video games for 72 hours straight. And so hopefully this antibody stick figure along with GAMED will help you guys remember the 5 different classes of antibodies. And so looking at this table right here, what I want you guys to notice is that, the light chain for all 5 classes of antibodies is exactly the same. So the light chain Y, there'd be the one that's Greek letter lambda. And so the light chain is not going to distinguish one antibody class from another. However, looking at the heavy chain, notice that each class of antibody has a unique heavy chain. And so it's the heavy chain that's going to distinguish one antibody from another antibody class from another. And so this first row of antibody, this first class of antibody is IgG and this is really the one that we've been talking about all along and so you can see the Y-shaped structure here that we talked about. Now, notice that IgE and IgD have very similar types of structures to IgG. However, IgA forms a dimer of these 2 Y-shaped structures and IgM tends to form a pentamer, containing 5 different Y-shaped structures here. Now, over here in this column, what we have is the primary feature of these antibodies. Now, IgG is actually the most prevalent and the most abundant antibody in our blood. So this is going to be one that's involved in protecting pretty much against all types of infections including bacterial and viral infections. Now, IgA is actually going to be highly concentrated in mucus membranes and it's going to be one that is typically secreted by our cells and it's also prevalent in our saliva. Now, IgM interestingly enough is usually going to be the first antibody that's going to be produced upon infection. So, IgM is going to be the one that's going to initiate the primary immune response. So the very first immune response. And then, the common second immune response would be IgG since it's so prevalent in our blood. Now IgE on the other hand here is gonna be one that's gonna defend against allergies or allergens. And so you can see here we have a guy who's saying I'm allergic to this kitty cat right here, and that's unfortunate because this is a cute little kitty cat. But IgE again is going to help defend against these allergens. Now IgD is one where its function is not really very well characterized. However, there are some textbooks and studies that say they're involved with activating B cells and allowing B cells to participate in immune responses. Now over here on the far right, what we have is the distribution of these antibody classes throughout our bodies. And so notice that pretty much all of the antibodies are going to be found in our bloodstreams, except for IgA, which again is going to be highly concentrated in our mucus membrane. So it's going to be lining our digestive systems a lot. And so this here concludes our introduction to the antibody classes and we'll be able to talk more about antibodies as we move along in our course. So, I'll see you guys in our next video.

Which of the following is NOT an immunoglobin class?

• A. IgM.
• B. IgH.
• C. IgG.
• D. IgE.
• E. IgD.

Answer: (B)

IgG

In this video, we're going to talk more details about the IgG class of antibody. And so the IgG antibody is the standard antibody. And that is because the IgG antibodies are the most abundant antibody class in our blood and in our tissues. And in some individuals, the IgG antibodies can make up to 80% of all of the immunoglobulins. Now these IgG antibodies can have a variety of different functions. However, they are capable of binding to very specific antigens, and they can eliminate pathogens by activating the complement system classical pathway, which recall, we covered the complement system classical pathway in our previous lesson videos when we discussed innate immunity. Now IgG antibodies are also well known for providing immunity to a developing fetus during pregnancy, as well as providing immunity to a newborn child after the child has been born. And so this is because these IgG antibodies have the amazing ability to cross the placenta between a mother and the baby. And so the mother's IgG antibodies are capable of crossing the placenta to protect the baby. So, literally the mother is protecting the baby with her antibodies. Now also, IgG antibodies are well known for having a long half life. And all that means is that, if it has a long half-life, is that it's very stable, and it is capable of lasting a long time within our bodies before it's broken down. And so this long half life actually extends the time of protection of a newborn through the first few months after birth until the baby is capable of generating its own antibodies. And so, basically what we're saying here is that, literally a mother's antibodies protect the baby as the baby is a developing fetus, and for the first few months of the baby's life until the baby's capable of generating its own antibodies. And if we take a look at our image down below, it's just an image emphasizing those important features of the IgG class of antibody. And so notice that its structure is going to be that standard antibody structure that we talked about in our previous lesson videos. Notice it is the most abundant class of antibodies. Around 80% of the antibodies are going to be IgG. It provides infant immunity during, again, pregnancy, as well as after childbirth as well because the IgG antibody has that amazing ability to cross the placenta between the mother and baby. And last but not least, this IgG antibody has many different immune functions including the ability to activate the complement system via the classical pathway where it can bind to antigens. And again, you can see activating C₃ convertase leading to many different types of complement system activities such as inflammation, opsonization, cell lysis of microbes, and so on. And again, if you don't remember these details here about the complement system classical pathway, be sure to go check out our older lesson videos on that content. But for now, this here concludes our brief lesson on the IgG class of antibodies and as we move forward in our course we'll also be able to discuss briefly some of the other classes of antibodies. So I'll see you all in our next video.

IgA

In this video, we're going to briefly discuss the IgA class of antibody. And so the IgA class of antibody is going to help protect mucous membranes throughout our bodies. And they can also be found in body secretions such as for example, saliva, tears, and breast milk. Now, the IgA antibody is commonly found as a dimer, which means that it consists of 2 identical subunits that are held together. And this dimer is referred to as secretory IgA or sIgA for short. And within this secretory IgA molecule, which is a dimer, it consists of 2 monomers that are held together by a peptide, and we'll be able to see that down below in our image. Now, the secretory IgA antibody in breast milk helps protect the intestinal tract of breastfed newborns which is why breastfeeding can actually be very healthy for newborn babies to provide them with those IgA antibodies that can help protect, again, the intestinal tract of those newborns. And so if we take a look at our image down below, we're just emphasizing those same important details of the IgA antibody. And, again, the IgA antibody is commonly found as a dimer that we refer to as secretory IgA. And so what you can see here is that there is a dimer. There's this unit right here and then there's this other identical unit right here. And these two units are held together by this blue peptide that is swirling around it. And so, that is the secretory IgA antibody. Again, this is really going to be important for protecting mucous membranes throughout our bodies and also it's going to be found in mucous, saliva, tears, and breast milk. And so you can see those images down below. And again, because it is found in breast milk it will help protect the intestinal tract of newborn babies that are breastfed. And so, you can see the IgA antibody here dissolved within this breast milk. And so, this here concludes our brief lesson on the IgA class of antibody. And again, we'll be able to get some practice applying these concepts and talk about the other classes of antibodies briefly as we move forward. So I'll see you all in our next video.

IgM

In this video, we're going to briefly discuss the IgM class of antibody. The IgM antibody is very important for controlling infections in the blood. It is actually the very first antibody that is going to be initially produced in a primary infection by plasma cells before a process known as class switching takes place. Later in our course, in a different video, we'll talk more about this antibody class switching. The IgM class of antibody is also the largest class of antibody in terms of its size and molecular mass because IgM antibodies consist of a pentamer, which means that it is composed of 5 identical Y-shaped subunits. Because it has 5 Y-shaped subunits, each subunit with 2 antigen binding sites, that means that there are 10 antigen binding sites. Because there are so many antigen binding sites on these IgM antibodies, that makes them very effective at linking antigens together. The large size of the IgM antibody class actually prevents the IgM antibody from leaving the blood, into the tissues. Thus, the primary role of the IgM antibody is to help control infections in the blood. Also, the IgM class of antibody is the most efficient antibody class at activating the complement system via the classical pathway.

If we take a look at our image down below, we'll have an image that helps to emphasize these same features of the IgM class of antibody. Notice first that it is indeed a pentamer that consists of these five identical Y-shaped subunits. That makes it the largest antibody class in size. Also, it is the very first antibody that is initially created by all plasma cells prior to the antibody class switching process that we'll get to talk about in more detail later in our course. The primary role of the IgM antibody, because of its large size, is to control infections in the bloodstream. This is the most effective antibody at activating the complement system. It activates the complement system when it binds to the antigen. Here we have a little image that shows you C3 convertase being generated upon activation of the complement system and then all of these actions that result from the complement system activation, including inflammation, opsonization, and cell lysis of microbes. If you don't remember much about the complement system, make sure to go back to our older lesson videos to check them out. This concludes our brief lesson on the IgM class of antibody, and we'll be able to get some practice applying these concepts and talk about the other classes as we move forward. So, I'll see you all in our next video.

IgE

In this video, we're going to talk briefly about the IgE class of antibody. The IgE antibodies are found on the surface of both basophils and mast cells. Recall that basophils and mast cells have a lot of similarities, but basophils circulate through the bloodstream, whereas mast cells are more localized within specific tissues.

When these IgE antibodies are on the surface of basophils and mast cells, they are capable of detecting and responding to very specific antigens. These IgE antibodies can cause the cell to undergo a process called degranulation, which you might recall from some of our previous lesson videos, just means that it will cause these cells to release their granules and the contents of their granules into the environment. This can include releasing inflammatory mediators that lead to inflammation in response to some kind of infection.

These IgE antibodies are very important for eliminating parasites, such as worms, and they also play an important role in the response to many different types of allergic reactions or hypersensitivities, which we'll talk more about allergic reactions and hypersensitivities much later in our course in a separate video. But for now, if we take a look at our image down below we can get a more solid understanding of this IgE class of antibody. Notice here it has that same antibody structure and is found on the surface of both basophils, as well as mast cells.

Upon binding their antigen, they cause degranulation, the release of the contents of the cytoplasmic granule. These IgE antibodies are important for protecting against parasitic infections like those caused by parasitic worms, as well as again responding to allergies or generating those allergic reactions. We'll talk more about allergies later in our course. But for now, this here concludes our brief lesson on the IgE class of antibody, and we'll be able to get some practice applying these concepts and then talk about the very last class of antibody. So, I'll see you all in our next video.

IgD

In this video, we're going to talk very briefly about the IgD class of antibody. IgD antibodies are found on the surface of B cells, and they signal B cell activation and maturation into an antibody-secreting plasma cell. These IgD antibodies are important for the development and maturation of the antibody response. If we take a look at our image down below, we can see that the IgD antibody is, again, a monomer. It makes up a very small percentage of all the antibodies, about 1% of all the antibodies. It can be found on the surface of B cells, as you see here. Their role on the surface of B cells is to help the B cell differentiate into plasma cells that can secrete antibodies. The IgD antibody helps activate the B cell and aids it in differentiating into a plasma cell. Ultimately, the plasma cell will be able to secrete a variety of different types of antibodies. This concludes our brief lesson on the IgD antibody and its role in the development and maturation of the antibody response, and we'll be able to get some practice applying this as we move forward. I'll see you all in our next video.

______ is the first immunoglobulin class produced during a primary response.

• A. IgA.
• B. IgE.
• C. IgM.
• D. IgG.

Answer: (C)

Which antibody class crosses the placenta from mother to child?

• A. IgA.
• B. IgE.
• C. IgM.
• D. IgG.

Answer: (D)

_______ is the immunoglobulin class that neutralizes viruses in the intestinal tract.

• A. IgG.
• B. IgA.
• C. IgD.
• D. IgE.

Answer: (B)

Which of the following antibodies is involved in causing basophils to release histamine when the antibody encounters an allergen?

• A. IgG.
• B. IgA.
• C. IgD.
• D. IgE.

Answer: (D)

The primary B-cell receptor is:

• A. IgD.
• B. IgA.
• C. IgE.
• D. IgG.

Answer: (A)

Review Table of Immunoglobin Classes

In this video, we're going to do a quick review of all the immunoglobulin classes or all the classes of antibodies. So we're going to do that by completing this review table of the immunoglobulin classes. What you'll notice is that we have these five classes of antibodies arranged so that they fit our mnemonic, which is GAMED. The IgG antibody is the very first one on this list. Notice that it is a monomer and it is the standard antibody. It has a very long half-life, which means that it's very stable and will last a long time within our bodies before it's broken down. It is the most abundant class of antibody. Around 80% of all antibodies will be IgG. It has this amazing ability to cross the placenta between the mother and the baby, which is going to help provide the fetus immunity during pregnancy as well as even after birth because these IgG antibodies last such a long time that the mother's IgG antibodies in the baby will also help to protect the baby even after birth. They're also important for activating the complement system which can lead to a variety of immune responses.

Now the IgA antibody, notice it is a dimer, and a lot of times we refer to this dimer as the secretory IgA antibody or sIgA antibody. Notice that its half-life is medium, and it's going to be important for protecting the mucous membranes. It's also found in many different types of body secretions including breast milk, which can help protect the infant's intestinal system when a mother breastfeeds her baby.

The next antibody that we have on here is the IgM antibody, which is the largest class of antibody. It is a pentamer, and its half-life is medium. It is the very first antibody that is initially created by all plasma cells prior to antibody class switching. It helps to control infections in the bloodstream. That is its main role, and it also is the most effective class at activating. Its half-life is very short and so it is going to be found on both the surfaces of basophils as well as mast cells, and they're important for triggering the release of inflammatory mediators causing these basophils and mast cells to degranulate, releasing their granules into their environment. The inflammatory mediators can cause inflammation, and the IgE antibodies also play a role in allergic reactions, and we'll get to talk more about that later in our course when we focus our attention on allergic reactions and hypersensitivities. They also play an important role in protecting against parasitic infections as well, like parasitic worms, for example.

Now last but not least, we have the IgD antibody, which is again a monomer and has a short half-life, which means that it does not last very long within the body. The IgD antibodies are going to be found on the surface of B cells and they play an important role in helping to trigger B cell activation and differentiation into antibody-secreting plasma cells. And so this here concludes our brief review of the 5 classes of antibodies, and we'll be able to get some more practice applying these concepts as we move forward. So I'll see you all in our next video.

Each class of antibody is specifically defined by its

• A. Amino acid sequence of the variable region of the light chain.
• B. Amino acid sequence of the constant region of the heavy chain.
• C. Ability to cross the placenta.
• D. Disulfide bonds.

Answer: (B)

Match the following antibody classes with their description: 
a) IgA.
b)  IgG.
c)  IgE.


___ First antibody produced during primary response. 
___ Protects mucous membranes.  
___ Most abundant antibody. 
___ Found on the surface of B cells. 
___ Triggers allergic reactions to allergens.

Okay, everyone. Here we have a great review question about the different classes of antibodies because we are going to match the antibody class with one of their descriptors. So we have our 5 different classes, and we know that these different antibodies do a variety of things, but let's try to figure out which one does each of these descriptions. So the first one is the first antibody produced during a primary response. In a primary response, your body is infected by a new pathogen that it has never seen before. And this is going to be the first antibody that is produced. This is going to be IgM. It is going to be the initial antibody produced by plasma cells before that plasma cell changes to a different class of antibodies. We call this class switching, and you will learn about this soon, but IgM is going to be produced first by all the plasma cells until they're told to switch to a different class. So that means that it is the first antibody produced. So that means we're going to put d for our first answer. Okay. So now we have protects the mucus membranes. Remember that this is going to be the antibody that is found in breast milk. And whenever babies breastfeed, it is going to protect them from pathogens that they may ingest because when they're first born, they have a weak immune system. So this antibody protects the mucus membranes in the digestive tract. And remember, this is IgA. So the correct answer to this one is going to be a. Alright. So now we have 3 left. The third one says the most abundant antibody. You might be thinking this is IgM because it's the first one to be made, but it's actually not. The most abundant antibody makes up about 80 percent of the antibodies in our body, and it's the only one that crosses the placenta between mother and baby and protects the infant for the first couple months of its life. And that is IgG. So the correct answer here is b. Okay. We have 2 left. The fourth one says found on the surface of B cells. It is the most common type of B cell receptor, and this antibody is involved in the differentiation and activation of B cells. This is going to be IgD. So that means answer choice e is the next answer. Excuse me. Now we have the last one, triggers allergic reactions to allergens. This antibody is going to activate mast cells and basophils, causing them to release histamine and trigger an inflammatory response, which is also known as an allergic reaction. And that is IgE, the only answer left. There we go. So that is going to be all of the different antibody classes and their major or most famous description or scenario that they perform. Okay, everyone. Let's move on to our next video.

Here’s what students ask on this topic:

What are the five classes of antibodies and their primary functions?

The five classes of antibodies are IgG, IgA, IgM, IgE, and IgD. IgG is the most abundant antibody in the blood and provides immunity to fetuses and newborns by crossing the placenta. IgA protects mucous membranes and is found in secretions like saliva and breast milk. IgM is the first antibody produced during an infection and is effective in the blood. IgE is involved in allergic reactions and defense against parasites. IgD aids in B cell activation and maturation. Each class has unique heavy chains that distinguish them from one another.

How does IgG provide immunity to fetuses and newborns?

IgG provides immunity to fetuses and newborns by crossing the placenta from the mother to the baby. This transfer of IgG antibodies helps protect the developing fetus during pregnancy and continues to provide immunity to the newborn for the first few months after birth. IgG has a long half-life, meaning it remains stable and effective in the baby's body for an extended period, offering protection until the baby can produce its own antibodies.

What is the role of IgA in the immune system?

IgA plays a crucial role in protecting mucous membranes throughout the body. It is commonly found in body secretions such as saliva, tears, and breast milk. In its dimeric form, known as secretory IgA (sIgA), it helps protect the intestinal tract of breastfed newborns. IgA prevents pathogens from adhering to and penetrating mucosal surfaces, thereby providing a first line of defense against infections in areas like the respiratory and gastrointestinal tracts.

Why is IgM considered the first antibody produced during an infection?

IgM is considered the first antibody produced during an infection because it is the initial antibody synthesized by plasma cells in response to a primary infection. It is a pentamer, meaning it consists of five identical Y-shaped subunits, making it the largest antibody class. Its large size and multiple antigen-binding sites make it highly effective at linking antigens together and activating the complement system, which helps control infections in the bloodstream.

What is the function of IgE in allergic reactions?

IgE plays a significant role in allergic reactions and defense against parasitic infections. It is found on the surface of basophils and mast cells. When IgE binds to specific antigens, it triggers these cells to undergo degranulation, releasing inflammatory mediators into the environment. This process leads to inflammation and other symptoms associated with allergic reactions. IgE is also important for eliminating parasites such as worms.