Review of the Complement System - Online Tutor, Practice Problems & Exam Prep

The complement system is activated through three pathways: the alternative pathway, the lectin pathway, and the classical pathway. Each pathway leads to the formation of C3 convertase, which cleaves the inactive protein C3 into C3a and C3b. C3a triggers inflammation, while C3b enhances opsonization, improving phagocytosis and facilitating microbe lysis via membrane attack complexes. This interplay between innate and adaptive immunity highlights the complement system's crucial role in immune responses against pathogens.

Review of the Complement System

In this video, we're going to review the complement system. And so because this video is a review, there's really no new information here. If you're already feeling really good about the complement system, then you can feel free to skip this video. But if you're looking for a good way to review a lot of the concepts that we talked about in our previous lesson videos, then stick around because this might be a helpful review for you.

In reviewing the complement system, we're going to examine the activation pathways, as well as the responses or the effects of activation of the complement system. Recall that the complement system consists of inactive proteins that are circulating in our blood and through the tissues. These inactive complement system proteins can become activated by one of three different pathways: the alternative pathway, the lectin pathway, or the classical pathway.

The alternative pathway activates the complement system and is triggered when C3b, a complement system fragment, binds to the surface of invading microbes. When C3b binds to these surfaces, it helps activate the complement system. The alternative pathway ultimately leads to the formation of the enzyme C3 convertase, where all three of these pathways converge.

The lectin pathway is activated through the MBL, the mannose-binding lectin. The mannose-binding lectin consists of a protein that binds to mannose carbohydrates found on the surface of microbes. When the mannose-binding lectin binds to the mannose on these microbes, it leads to a cascade of events that results in the formation of C3 convertase.

The classical pathway is triggered and activates the complement system when antibodies from the adaptive immune system bind to antigens on the surface of microbes. These antibodies lead to a cascade of events that ultimately results in the formation of C3 convertase. The classical pathway has an asterisk next to it because it illustrates why the complement system is called the complement system. It complements adaptive immunity through the interaction of antibodies, which are part of adaptive immunity, triggering the complement system, a component of innate immunity, showcasing the crosstalk between innate and adaptive immunity through the classical pathway.

Regardless of which pathway is used, the alternative, lectin, or classical pathway, they all lead to the formation of C3 convertase. This enzyme is represented as scissors here because it cleaves the inactive complement system protein called C3 into C3a and C3b. There will be high levels of C3a and C3b after the formation of C3 convertase. C3a, along with other complement system proteins such as C5a, can trigger the inflammatory response to help eliminate microbes. C3b acts as an opsonin to improve the effectiveness of phagocytosis, allowing phagocytes like macrophages to phagocytose microbes more easily and effectively. Additionally, C3b can interact with a series of other complement system proteins to lead to microbe cell lysis through the formation of membrane attack complexes that form pores in the membranes of gram-negative bacteria.

This concludes our review of the complement system, and we’ll be able to apply some of the concepts in this review as we move forward. See you all in our next video.

Which of the following statements about the complement system is true?

• A. There are many methods of triggering the complement system.
• B. There are many immune responses triggered by the complement system.
• C. All triggers of the complement system lead to the formation of C3 convertase.
• D. C3a and C3b result in different immune response for the complement system.
• E. All of the above are true.

Which complement molecule is an attractant for neutrophils?

• A. C4b.
• B. C5a.
• C. C3b.
• D. C7a.

The complex resulting from complement activity that leads to cell lysis is the

• A. Prostaglandin complex.
• B. Leukotriene activating complex.
• C. Membrane attack complex.
• D. Histamine complex.

Which of the following are most susceptible to complement lysis?

• A. Gram-positive bacteria.
• B. Gram-negative bacteria.
• C. Bacteriophages.
• D. Prions.

Here’s what students ask on this topic:

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. The alternative pathway is triggered by the binding of C3b to the surface of invading microbes. The lectin pathway is activated when mannose-binding lectin (MBL) binds to mannose carbohydrates on the surface of microbes. The classical pathway is initiated when antibodies from the adaptive immune system bind to antigens on the surface of microbes. All three pathways converge at the formation of C3 convertase, which cleaves the inactive protein C3 into C3a and C3b, leading to various immune responses.

How does the classical pathway of the complement system work?

The classical pathway of the complement system is activated when antibodies from the adaptive immune system bind to antigens on the surface of microbes. This binding triggers a cascade of events that ultimately leads to the formation of C3 convertase. C3 convertase then cleaves the inactive complement protein C3 into C3a and C3b. C3a triggers inflammation, while C3b acts as an opsonin to enhance phagocytosis and can also lead to the formation of membrane attack complexes that lyse microbial cells. This pathway highlights the interplay between innate and adaptive immunity, as it involves antibodies from the adaptive immune system to activate the complement system, which is part of innate immunity.

What role does C3 convertase play in the complement system?

C3 convertase is a crucial enzyme in the complement system that is formed through the activation of any of the three pathways: alternative, lectin, or classical. Its primary role is to cleave the inactive complement protein C3 into two fragments: C3a and C3b. C3a is involved in triggering the inflammatory response, which helps to eliminate microbes. C3b acts as an opsonin, enhancing the effectiveness of phagocytosis by making it easier for phagocytes like macrophages to engulf microbes. Additionally, C3b can interact with other complement proteins to form membrane attack complexes that create pores in microbial cell membranes, leading to cell lysis.

What is the function of C3a and C3b in the complement system?

In the complement system, C3a and C3b have distinct but complementary roles. C3a primarily functions to trigger the inflammatory response, which helps to recruit immune cells to the site of infection and eliminate microbes. On the other hand, C3b acts as an opsonin, which means it tags microbes for easier recognition and ingestion by phagocytes like macrophages. Additionally, C3b can interact with other complement proteins to form membrane attack complexes that create pores in the membranes of microbial cells, leading to their lysis. Together, C3a and C3b play crucial roles in enhancing the immune response against pathogens.

How does the complement system interact with both innate and adaptive immunity?

The complement system serves as a bridge between innate and adaptive immunity. It is part of the innate immune system but can be activated by components of the adaptive immune system, such as antibodies. This interaction is most evident in the classical pathway, where antibodies from the adaptive immune system bind to antigens on the surface of microbes, triggering the complement cascade. This crosstalk enhances the overall immune response, as the complement system can amplify the effects of antibodies, leading to more effective pathogen elimination through processes like opsonization, inflammation, and cell lysis.