This lesson will cover the five different classes, or isotypes, of antibodies. We will cover IgA, IgE, IgG, IgM, and IgD, as well as their basic structure and function.
There are five different isotypes, or classes, of antibodies. Each isotype differs in its constant region of the heavy chain. This is not to be confused with variation of the variable regions of antibodies. Those are the regions antibodies use to bind to specific antigens. As a class, we are going to take a little field trip to a beautiful and scenic field to learn about these five isotypes.
Immunoglobulin G and M
As we arrive in our school bus to our beautiful field, the first thing we see is a field full of green grass. This green grass is representative of Immunoglobulin G, or IgG for short. IgG is the most common antibody found in serum and the only one that crosses the placenta.
This means that the only antibody class, or isotype, found in fetal circulation is IgG. In addition, IgG makes up about 80% of all of the antibodies found in serum.Like grass, it’s pretty darn common. In addition, IgG is the stereotypically Y-shaped looking antibody.
It has two arms, which bind to an antigen, and one tail, the Fc region, which can be used for identification by our white blood cells.As we continue to scan the beautiful field, we notice that there are giant maple trees standing here and there. These big maple trees represent Immunoglobulin M, or IgM for short. This is the largest class of antibody and the first to be produced during an immune response. IgM is truly gigantic. You can basically think of this antibody as five IgG molecules stuck together in a circle. That means there are a total of not two, but ten binding sights!
Immunoglobulin A, D, and E
Besides maple trees, another type of tree we see scattering the field is the elm tree.
Allergy sufferers know this tree all too well. The elm tree releases lots of pollen that wreaks havoc upon many people. Those allergic reactions have a lot to do with Immunoglobulin E, or IgE for short. IgE is the isotype of antibody that is rarest in the serum and is mostly found bound to mast cells and basophils via its Fc region.The mast cells and basophils are white blood cells involved in many things, notably allergic reactions and inflammation.
When an allergen binds to the IgE found on the surface of these cells, it signals the cell to release all sorts of molecules that cause the typical inflammatory responses, such as congestion, itching, redness, and so on.Since the elm trees are causing so much trouble, let’s quickly move away from them. As we do so, we come up against an arroyo. These are dry creek beds that fill with water after heavy secretions from the sky, that is to say, after a lot of rain. Our arroyo is a metaphor for Immunoglobulin A, or IgA for short. This is the isotype of antibody most commonly found in bodily secretions. For example, it can be found in:
It’s more important than being just the most commonly found antibody in bodily secretions.
There’s a reason for all of this. For example, your upper respiratory tract has mucous membranes to which viruses and bacteria love to attach. Well, it’s the job of IgA to bind to these viruses and bacteria and prevent their attachment and entry into the cells of your mucous membranes and beyond.
Last, but certainly not least, as we cross over the arroyo, we see that on the other side are some deer. We see that the deer are quite young and still maturing. Our Immunoglobulin D, or IgD, is the class of antibody found on B-cells that is involved in their maturation and activation. Basically, IgD helps our little deer grow up a little bit more by exposing them to the world around them.
One last thing you must know before we leave the field is that IgD, IgE and IgG are all called monomers and have the characteristic shape of a Y, with two binding sites. This is in contrast to IgA, which is a dimer, or combination of two monomers, which gives it four binding sites. And, as previously mentioned, it is also in very stark contrast to IgM, which is a pentamer, or a combination of five monomers, which gives it ten binding sites.
That’s why our maple trees are so big!
Now that we’ve left the field and are on our bus ride back to school, let’s review everything we saw today.Immunoglobulin G is the most common antibody found in serum and the only one that crosses the placenta.Immunoglobulin M is the largest class of antibody and the first to be produced during an immune response.Immunoglobulin E is the isotype of antibody that is rarest in the serum and is mostly found bound to mast cells and basophils, whereas Immunoglobulin A is the isotype of antibody most commonly found in bodily secretions.
Finally, Immunoglobulin D, or IgD, is the class of antibody found on B-cells that is involved in their maturation and activation.It’s all pretty simple to recall. The grass was the most common thing we saw, the maple trees were huge, the elm trees gave us all allergies, the arroyo was waiting for rain to be secreted out of the clouds, and the deer were only just maturing.
After this lesson, you’ll be able to:
- Describe the function of the five different isotypes of antibodies
- Identify the most common antibody found in serum
- List the antibodies that are classified as monomers
- Explain the unique structural characteristic of IgM