In this video, we're going to continue to talk about how some pathogens can avoid phagocytosis by talking specifically about how some pathogens can use capsules and M proteins to avoid opsonization. And so first we need to recall from some of our previous lesson videos what the term Opsonization means. And so recall that Opsonization refers to the process that makes microbes easier to bind or engulf during phagocytosis. And so once again, some pathogens can prevent opsonization. By preventing opsonization, they make phagocytosis more difficult and therefore they can avoid phagocytosis. Some pathogens are able to prevent opsonization by the production of a capsule. Recall from some of our previous lesson videos that some bacteria have a bacterial capsule, which is an outer layer that surrounds the cell and consists of carbohydrates and proteins in some cases. These capsules can prevent immune cells and opsinins like the complement protein C3b from adhering to the pathogen. By preventing immune cells and opsinins from adhering to the pathogen, that will avoid opsonization and again avoid phagocytosis.
Now, very similar to capsules, the production of the cell wall protein called the M protein can also allow a pathogen to avoid opsonization. What happens is the M protein will bind to another regulatory protein or inhibitory protein that is going to be degrading the enzyme C3 Convertase which is important for complement system activation. By degrading C3 Convertase, that is going to inhibit or block the production of C3b, the complement protein C3b, which again we know acts like an opsinin. If we take a look at our image down below, we'll be able to get a better understanding of this idea. Notice on the left hand side, we're showing you how capsules can block C3b binding. Notice that we have two microbes. We have this microbe on the left that is in blue, and then we have the microbe that's on the right. Notice that the microbe that is on the left does not have a capsule. Because it does not have a capsule, the opsonin protein C3b is capable of binding to the surface of that microbe, and therefore opsonization will take place and that means that this microbe is likely to be engulfed through the process of phagocytosis. However, the microbe on the right, notice, has a bacterial capsule, which is this outer layer that's represented in yellow, and that outer layer again consists of carbohydrates and proteins. This outer capsule can block the binding of C3b. So C3b cannot bind. If C3b cannot bind, what that means is that there is no opsonization. By preventing opsonization with the capsule, that is going to allow it to avoid phagocytosis.
Now, on the right hand side, we're showing you how the M protein can block C3 convertase. What you'll notice is that the cell over here on the left is producing this M protein on its cell wall. This M protein will associate with an inhibitory protein, this little purple structure or a regulatory protein, and the inhibitory protein will basically degrade C3 convertase. You might recall when we covered the complement system in some of our previous lesson videos that C3 convertase is an enzyme that is important for complement system activation. By degrading C3 Convertase, C3 Convertase is going to be blocked, and it will be blocked from making the opsinin C3b, from C3. Usually what happens is C3 convertase will be involved with converting C3 into C3a and C3b and then C3b would serve as an opsinin. However, if there is M protein, that means it will associate with the inhibitory protein. The inhibitory protein blocks C3 convertase, and then what that means is C3 will not be converted into C3b. By blocking C3 convertase, it is blocking the production of an opsinin, and that is going to avoid opsonization, and that will avoid phagocytosis.
This here concludes our brief lesson on how pathogens can use capsules and M proteins to avoid opsonization and therefore avoid phagocytosis. We'll be able to get some practice applying these concepts and learn more as we move forward. So I'll see you all in our next video.