Introduction to the Complement System - Online Tutor, Practice Problems & Exam Prep

The complement system consists of inactive proteins in the blood that activate during infections, triggering an immune response. Activation occurs through three pathways: alternative, lectin, and classical, leading to the formation of C3 convertase, which cleaves C3 into C3a and C3b. These fragments promote opsonization, cell lysis, and inflammation, enhancing innate immunity. The classical pathway complements adaptive immunity by utilizing antibodies. Understanding these mechanisms is crucial for grasping immune responses and their roles in pathogen elimination.

Complement System

In this video, we're going to begin our introduction to the complement system. And so, the complement system is really just a system of inactive proteins that are found circulating in our blood and in our tissues, and they are capable of initiating an immune response when these inactive complement system proteins become activated by an infection. And so once again, these complement system proteins start off as inactive proteins. But upon signals of microbes during an infection, those inactive proteins can become activated proteins. And the activated complement system proteins will lead to a cascade of several different reactions that ultimately generate some kind of immune response. And the immune response that can be triggered by activation of the complement system includes removal of invading microbes and inflammation, which can also help to eliminate microbes.

Now it is important to note that the complement system is not adaptable, which means that the complement system does not change or adapt over time through exposures to different microbes, which means that the complement system is not part of adaptive immunity. And so the complement system instead is considered part of innate immunity. And, it is important to note that however, although the complement system is part of innate immunity, it does complement or act in combination with antibodies, and with the antibody response of adaptive immunity. And so this complement system, although it is part of innate immunity, it does complement or act in combination with adaptive immunity. And so that is an important feature to keep in mind.

Now the complement system, once again, consists of these inactive proteins. And, the complement system proteins can be activated by one of three different pathways that we refer to as the alternative pathway, the lectin pathway, and the classical pathway. And so if we take a look at our image down below, notice we're showing you our map of the lesson on innate immunity. And we've already talked about the first line defenses, and we know that we've talked about the second line of defense and cells of immunity. And, we've talked about scanning systems including cell communication and pattern recognition receptors. So here in this video, we're focusing specifically on the complement system. And the complement system proteins again are inactive proteins that can be activated by one of three different pathways. The alternative pathway, the lectin pathway, and the classical pathway. Now it turns out that it's this classical pathway over here that is going to complement adaptive immunity. And so, notice that it does utilize antibodies and we'll get to talk a lot more about each of these three different types of complement system activation pathways as we move forward in our course.

Now another thing that's important to note is that the complement system, because it consists of these inactive proteins, it does serve somewhat as a scanning system. So it is going to be important for sensing and detecting signs of microbes. However, upon activation of the complement system proteins, those complement system proteins can trigger innate effector action such as, for example, phagocytosis and inflammation. And so the complement system we're saying here, can promote or promotes phagocytosis and inflammation. And we'll be able to talk more about this idea as we move forward in our course. But for now, this here concludes our brief introduction to the complement system, and we'll be able to learn a lot more about the complement system as we move forward in our course. So I'll see you all in our next video.

What does the word “complement” mean in the name complement system?

• A. This is a system of proteins that complements or enhances the abilities of the immune system to kill microbes.
• B. This is a system of proteins that replaces the innate immune system when the infection is too extreme.
• C. This is a system of proteins that competes with the adaptive immune system to kill pathogens.

Which of the following are results of activation of the complement system? 

1. Removal or destruction of microbes. 2.  Inflammation. 3.  Production of antibodies.

• A. 2 only.
• B. 1 and 2.
• C. 3 only.
• D. 1, 2, and 3.

Proteins of the Complement System

In this video, we're going to briefly talk about the proteins of the complement system. The complement system proteins are all designated with a letter "C", and they are numbered C1 through C9 in terms of the order of their discovery, where C1 was first and C9 was discovered last. These complement system proteins remain active. They circulate through our blood and in our tissues in their inactive forms until they are fragmented. Once these complement system proteins are fragmented, they become activated. The activated fragments are indicated by lowercase letters, such as, for example, the lowercase letter "a" and the lowercase letter "b".

The complement system protein C3 is a very important and critical complement system protein. These complement system proteins exist in their inactive forms. C3 is a critical inactive complement system protein. The C3 complement system protein can become activated by the activity of an enzyme called C3 convertase. C3 convertase is an enzyme that activates C3 by hydrolyzing or fragmenting C3 into the C3_a and C3_b fragments. The C3_a and C3_b fragments, which are active, can interact with other complement system components, ultimately causing some kind of immune response. Recall that the immune responses of complement system activation include opsonization, cell lysis, and inflammation, all of which we'll talk more about as we move forward in our course.

If we take a look at our image down below, notice what we're showing you here at the top is the enzyme C3 convertase, which is being represented here as this pair of scissors because its function is to enzymatically cleave or fragment C3. Here we have the complement system protein C3 right in the middle. C3 convertase is an enzyme that will cleave or fragment C3 into the activated fragments C3_a and C3_b. C3_a and C3_b through a cascade of reactions can ultimately lead to these immune responses. For example, C3_a can lead to inflammation, an innate immune response that can help to eliminate microbes. We'll get to talk a lot more details about inflammation later in our course. And C3_b can lead to a process known as opsonization, which we'll get to talk more about moving forward in our course as well, but it helps to improve phagocytosis, and it can also lead to cell lysis. Here we're showing you a microbe that is being lysed. These proteins of the complement system need to become activated. Upon activation, they can lead to immune responses such as inflammation, opsonization, or cell lysis.

This here concludes our brief lesson on the proteins of the complement system, and we'll be able to learn more about the complement system as we continue to move forward. So I'll see you all in our next video.

If the complement proteins are always present in the body, then why are the results of the complement system not always occurring?

• A. The complement system proteins need to be activated by the invasion of a microbe.
• B. The complement system proteins are held in the lymph nodes until an infection occurs.
• C. The complement system proteins are inactive until cleaved by another complement protein.
• D. The complement system proteins are not always present and only translated when an infection occurs.
• E. A and B.
• F. A and C.
• G. D only.

Place the steps of the complement system in the correct order (Order steps 1-3). 

_____ Opsonization, cell lysis of microbes, or inflammation occurs. 

_____ C3 convertase splits C3 into C3a and C3b. 

_____ C3a and C3b recruit other complement proteins to create an immune response.

Okay, everyone. Here we have our next practice problem. Place the steps of the complement system in the correct order from 1 to 3. Okay. So, the Complement System. What do we know about it? We know it's a system of proteins and they are there to enhance the effects of the immune system and they help with killing and removing microbes and triggering inflammation. Now, what do we know? What's the first thing that's going to happen? Well, actually, let's just look at the three things and see if we can figure out what's the order. So the first one that we have, this one right here, says, opsonization, cell lysis of microbes, or inflammation occurs. Well, that kind of sounds like the results of inflammation. So, I'm thinking that's going to be the very last one. So I'm gonna put a 3 here. Because these are the results. This is the end result, what we want to happen when the complement system is acting with the immune system. One of these three things is going to occur when the complement system is activated. Okay. So what happens before that? So answer choice well, not answer. The second one says, C3 convertase splits into C3a and C3b. Okay. So we know that C3 convertase cuts the C3 protein and we make a C3a and a C3b. But is that the first thing to happen or the second thing to happen? Let's read the very last one. C3a and C3b recruit other complement proteins to create an immune response. Okay. So at this point, C3a and C3b have already been created. So that means that this one must be the first step because this is creating C3a and C3b and that means this is the second step. So the order goes, C3 convertase splits C3 in half, and creates C3a and C3b. That's step 1. Step 2 is C3a and C3b then collect other proteins, and they're going to say, hey, let's cause an immune response. Then the third thing that happens is the actual immune response, whether that's opsonization, cell lysis, or inflammation. Any of those three things can happen. So that is the order of the steps of the complement system. Okay, everyone. Let's move on to our next video.

Map of the Lesson on the Complement System

In this video, we're going to introduce our map of the lesson on the complement system, which is down below right here. And so we can go ahead and label the title of this map the complement system. Now, activation of the complement system can actually be initiated by one of three different pathways, which are the alternative pathway, the lectin pathway, or the classical pathway. Now each of these three different pathways does differ in the initiation of how they go about activating the complement system. However, although they differ in the initiation, all three pathways end up resulting in the formation of the enzyme C₃ convertase. And recall from our last lesson video that C₃ convertase is an enzyme that is capable of activating the complement protein C₃ by fragmenting it into C_3a and C_3b. And, of course, we know that C₃ convertase can end up resulting in three possible effects or three possible immune responses, which are opsonization, cell lysis of the invading microbe, or the inflammatory response leading to inflammation.

As we move forward in our course, we're going to talk about these three different pathways that activate the complement system, and we'll also get to talk about these three effects that can result from activation of the complement system. And so, if we take a look at our image down below, notice that the complement system again consists of these inactive proteins that circulate in the blood and in our tissues. These inactive complement system proteins can become activated through one of three different pathways: the alternative pathway, the lectin pathway, or the classical pathway. Notice that the classical pathway here has this little asterisk because the classical pathway is the one that complements adaptive immunity. The classical pathway is going to utilize antibodies, which helps it complement adaptive immunity.

Notice that all three pathways - alternative, lectin, and classical pathways - lead to the formation of the enzyme C₃ convertase. And we know from our last lesson video, once again, that C₃ convertase will enzymatically cleave C₃ into C_3a and C_3b, which can ultimately lead to these three immune responses: opsonization or improved phagocytosis, microbe cell lysis, or the inflammatory response. And so, those are the effects of activation of the complement system. This here again concludes our map of the lesson on the complement system. But as we move forward, we're going to talk more details about all of these different things that you see here. So I'll see you all in our next video.

How do the three pathways of the complement system differ?

• A. The pathways differ in the resulting immune response they trigger.
• B. The pathways differ in the complement proteins that they activate.
• C. The pathways differ in what triggers the initiation of the pathway.

The possible effects of the lectin pathway of the complement system are?

• A. C3 convertase formation.
• B. Opsonization of infecting microbes.
• C. Cell lysis of infecting microbes.
• D. Triggering the inflammatory response.
• E. All of the above.

Here’s what students ask on this topic:

What is the complement system and how does it function in the immune response?

The complement system is a network of inactive proteins circulating in the blood and tissues, which activate in response to infections. These proteins, once activated, initiate a cascade of reactions that enhance the immune response. Activation occurs through three pathways: the alternative, lectin, and classical pathways. The classical pathway involves antibodies and complements adaptive immunity. The activation of these pathways leads to the formation of the enzyme C3 convertase, which cleaves the protein C3 into C3a and C3b. These fragments promote opsonization (enhancing phagocytosis), cell lysis (breaking down microbial cells), and inflammation, all of which help eliminate pathogens.

What are the three pathways that activate the complement system?

The complement system can be activated through three distinct pathways: the alternative pathway, the lectin pathway, and the classical pathway. Each pathway differs in its initiation mechanism. The alternative pathway is triggered by microbial surfaces, the lectin pathway is activated by mannose-binding lectin binding to microbial carbohydrates, and the classical pathway is initiated by antibodies binding to antigens. Despite these differences, all three pathways converge to form the enzyme C3 convertase, which cleaves C3 into C3a and C3b, leading to immune responses such as opsonization, cell lysis, and inflammation.

How does the classical pathway of the complement system complement adaptive immunity?

The classical pathway of the complement system complements adaptive immunity by utilizing antibodies. When antibodies bind to antigens on the surface of a pathogen, they form immune complexes that activate the classical pathway. This activation leads to the formation of C3 convertase, which cleaves C3 into C3a and C3b. These fragments enhance the immune response by promoting opsonization, cell lysis, and inflammation. Thus, the classical pathway bridges innate and adaptive immunity, leveraging the specificity of antibodies to target and eliminate pathogens more effectively.

What roles do C3a and C3b play in the immune response?

C3a and C3b are fragments produced by the cleavage of the complement protein C3 by the enzyme C3 convertase. C3a primarily functions as an anaphylatoxin, promoting inflammation by increasing vascular permeability and attracting immune cells to the site of infection. C3b, on the other hand, acts as an opsonin, binding to the surface of pathogens and enhancing their recognition and ingestion by phagocytes. Additionally, C3b can participate in the formation of the membrane attack complex (MAC), leading to the lysis of microbial cells. Together, C3a and C3b play crucial roles in amplifying the immune response and eliminating pathogens.

What is the role of C3 convertase in the complement system?

C3 convertase is a crucial enzyme in the complement system that catalyzes the cleavage of the complement protein C3 into its active fragments, C3a and C3b. This enzyme is formed through the activation of any of the three complement pathways: alternative, lectin, or classical. The cleavage of C3 by C3 convertase is a pivotal step, as it leads to the generation of C3a and C3b, which are essential for promoting opsonization, cell lysis, and inflammation. Therefore, C3 convertase plays a central role in amplifying the immune response and facilitating the elimination of pathogens.