in this video we're going to talk about how some pathogens have evolved mechanisms of avoiding the complement system of innate immunity. And so recall from some of our previous lesson videos that we've already discussed the complement system in detail. And so recall from those videos that complement system activation can result in inflammation optimization or membrane attack complex is or M. A. C. S. And so activation of the complement system will lead to the destruction and elimination of the pathogen. Now some bacteria are resistant to the complement system and they're resistant to the complement system because they can prevent activation of the complement system. And so these bacteria that are resistant to the complement system are specifically called serum resistant bacteria. And so the term serum refers to the specific component of the blood where the complement proteins reside. Now, serum resistant bacteria are able to prevent Complement system activation and therefore they're able to prevent their own destruction and elimination and they're able to do that by binding and inhibiting the complement system protein c. three b. And so recall from our previous lesson videos that c. three B. is a complement system protein that's important for activating the remote the rest of the complement system. And so the regulatory proteins on serum resistant bacteria are going to be the ones that bind and inhibit C. Three B. And by binding and inhibiting C. Three B. It prevents C three convert its formation and that will ultimately prevent activation of the complement system. And so if we take a look at our image down below, we can get a better understanding of the complement system and serum resistant bacteria or complement system activation and how that relates to serum resistant bacteria. And so notice that our image here is broken up into really two regions the region the top region with the greenish background and then the bottom region down below with the reddish background. And notice that with the top we're looking at a scenario where there is no regulatory protein, this is going to be a cell without the regulatory protein. So this would just be a normal bacterial cell, it is not going to be a serum resistant bacterial cell. And so without this regulatory protein, C three B is able to bind to the bacterial cell surface and then it will associate with other complement system proteins. And when C three B binds with other complement system proteins and form C three converts, C three converts will lead to the activation of the complement system. And of course activation of the complement system is going to lead to either uh inflammation optimization or membrane attack complex is and this bacterial cell would be eliminated. However, down below with the reddish background, what we're looking at are the serum resistant bacteria, which is really what most of this text here is focusing on. And what you'll notice about these serum resistant bacteria is they is that they do actually have a regulatory protein. They they this is a scenario with the regulatory protein. And so notice that the regulatory protein is a protein on the surface of specifically serum resistant bacterial cells. And so what the serum regulatory protein will do is uh the serum resistant regulatory protein will do is it will bind to C. Three B. And block and inhibit C. Three B. And prevent C. Three B. From interacting with the other complement proteins. And so wen C three B is bound by the regulatory protein. Again, the other complement system proteins that will not be able to interact with it. And so C three B will be blocked and it will not be able to form C. Three convert taste. And if C three convert taste is not formed then there will be no activation of the complement system. And if there's no activation of the complement system, that means that um inflammation optimization and membrane attack complex is will not be a result. And that means that the serum resistant bacteria is able to resist the complement system and prevent activation of the complement system as we discussed. And so this here concludes our lesson on how some pathogens are able to avoid the complement system, specifically the serum resistant bacteria. And we'll be able to get some practice applying these concepts as we move forward. So I'll see you all in our next video