This lesson will cover the basic but important structural components and sites of an antibody as well as their function.
This includes heavy chains, light chains, variable domains, constant domains, paratopes, and epitopes.
Antibodies are little glycoprotein molecules that are the major components of the humoral immune system. They aren’t just little globs of nothing, however. They are highly specialized structures created to go after very specific invaders in your body. If it wasn’t for antibodies and their unique shape, we’d have a very hard time fighting off disease.
Chains and Domains
To me, antibodies, also called immunoglobulins, sort of look like one of those old-fashioned dowsing rods people believe can help find water. While antibodies don’t find water, they do find specific pathogens thanks to their unique structure.
Each Y-shaped antibody is made up of four chains of protein. There are two heavy chains and two light chains that are, once again, polypeptides that combine to form an antibody.Each part of a chain, heavy or light, has what’s known as a constant domain. This is the part of the antibody that is the same across a unique isotype of immunoglobulin.
An isotype is just a different variation of antibody. Therefore, each isotype of immunoglobulin would have a different constant domain in its heavy chain. However, there needs to be variation within an isotype of immunoglobulin for maximal effectiveness of your immune system.Therefore, each part of every light and heavy chain has what’s known as the variable domain. This is the region located at the end of the light and heavy chains of an antibody.
This region allows for each individual isotype of antibody to be very specific to a type of antigen. This means that when a white blood cell, called a plasma cell, produces an antibody for a specific pathogen, it will alter the variable region such that the antibody will be able to attach to a very specific pathogen, but not another.
Paratope and Epitope
Let’s use an example to solidify what I mean. Any type of foreign substance in your body, such as a virus, bacteria, fungi, toxins, drugs, and so forth, can trigger a very similar response.
But we’ll focus on bacteria for now.If a bacterium, a Lego piece, enters your body, it’s bound to have a surface receptor on it. The receptors are like those little round things sticking off the top of the Lego pieces. These receptors are used by the bacterium to communicate, eat, and so forth. However, these receptors can also be used by our immune system.
Our body isn’t a bacterium; it’s not made up of Legos. Hence, it probably won’t have the same receptors as a bacterium. Therefore, components unique to that bacterium, such as its receptors, can be used by our immune system as a signal to destroy the bacterium.The ability of an antibody to attach to only one specific part of a unique pathogen but not to another is a result of the variable domain of the antibody.
When the antibody is made by a plasma cell, the variable domain is custom-fit, so to speak, to the specific pathogen we were talking about. The part of the variable domain of an antibody that recognizes and binds to an antigen is called a paratope and is like a lock. The part of an antigen that is recognized and bound by an antibody is called an epitope and is like a key.Don’t get confused between a pathogen, antigen, and epitope. A pathogen is the entire cell, the Lego piece, or a bacterium. The antigen is some kind of foreign protein or polysaccharide that helps to make up the cell, like a receptor on the surface of a bacterium, or those round things on the Lego pieces.
The epitope is the itty-bitty little portion of the antigen, in this case a receptor, which is recognized and bound by an antibody. It’s like that tiny, barely readable but very recognizable Lego brand-name that is written on top of the round bumps on the Lego piece itself. Once an antibody recognizes and binds an epitope it is by extension bound to the receptor, our antigen, which is obviously bound to the surface of the pathogen, our bacterium. This type of binding allows an antibody to tag a very large foreign invader for destruction by other components of the immune system just by binding a very tiny portion of it.
For this lesson, you should recall that each antibody is made up of four chains of protein. There are two heavy chains and two light chains that are, once again, polypeptides that combine to form an antibody.Each part of a chain, heavy or light, has what’s known as a constant domain (this is the part of the antibody that is the same across a unique isotype of immunoglobulin) and a variable domain, which is the region located at the end of the light and heavy chains of an antibody.The part of the variable domain of an antibody that recognizes and binds to an antigen is called a paratope and is like a lock, while the part of an antigen that is recognized and bound by an antibody is called an epitope and is like a key.
After you have completed this lesson, you should be able to:
- Define antibodies and describe their function
- Recall how many chains of protein make up each antibody
- Discuss constant and variable domains
- Differentiate between paratopes and epitopes