In this video, we're going to briefly discuss the lectin pathway, which is the second pathway of activating the complement system. The lectin pathway activates the complement system using pattern recognition molecules that are called mannose binding lectins. Now, these mannose binding lectins are commonly abbreviated as MBLs, and so these mannose binding lectins or MBLs are lectins themselves. Recall from our previous lesson videos that lectins are proteins that bind carbohydrates. And so these mannose binding lectins or MBLs are lectins that bind to specific arrangements of mannose, which is a specific type of carbohydrate. This mannose carbohydrate is commonly found on bacterial and fungal cell surfaces. These MBLs will bind to the mannose on the cell surface of bacteria and fungi. These bound MBLs will interact with fragments of activated complement proteins, ultimately forming what we call the C₃ convertase enzyme. We know that the C₃ convertase enzyme is going to cleave C₃ into C_3b and C_3a, and that will lead to immune responses such as inflammation, cell lysis of the microbes, or opsonization.

Notice over here on the left-hand side, we're showing you our map of the complement system, and it can be activated by the lectin pathway, which uses these mannose binding lectins or MBLs. That will ultimately lead to the formation of C₃ convertase as we just discussed. Over here on the right-hand side, we're showing you the lectin pathway, which again is going to be using these MBLs, these mannose binding lectins, which again are these proteins that you can see right here in this image that binds to mannose on the surface of microbes.

The mannose in this image is being represented by this little green structure. Again, it is found on the surface of microbes, and so when the mannose binding lectin binds to the mannose, again, it is capable of activating this lectin pathway, ultimately leading to a cascade of events that leads to the formation of C₃ convertase.

This here concludes our brief lesson on the lectin pathway and how it activates the complement system using MBLs. We'll be able to get some practice applying these concepts as we move forward, and then we'll get to talk about the classical pathway of complement system activation. So, I'll see you all in our next video.