Review of the Complement System: Videos & Practice Problems

The complement system is a crucial part of innate immunity, activated through three pathways: the alternative, lectin, and classical pathways. Activation 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. Additionally, C3b contributes to the formation of membrane attack complexes, leading to cell lysis of pathogens. This interplay between innate and adaptive immunity highlights the complement system's role in immune responses against invading microbes.

Review of the Complement System

The complement system is a crucial part of the immune response, consisting of inactive proteins that circulate in the blood and tissues. It can be activated through three main pathways: the alternative pathway, the lectin pathway, and the classical pathway. Each pathway leads to the formation of C3 convertase, an essential enzyme in the complement cascade.

The alternative pathway is initiated when C3b, a fragment of the complement protein, binds to the surface of invading microbes. This binding triggers a series of reactions that ultimately form C3 convertase. The lectin pathway, on the other hand, is activated by mannose-binding lectin (MBL), which attaches to mannose carbohydrates on the surface of microbes. This interaction also leads to the formation of C3 convertase through a cascade of events.

The classical pathway is distinct as it involves antibodies from the adaptive immune system binding to antigens on microbes. This pathway exemplifies the synergy between innate and adaptive immunity, as the complement system enhances the immune response initiated by antibodies. Regardless of the pathway, the end result is the activation of C3 convertase, which cleaves the inactive complement protein C3 into two fragments: C3a and C3b.

C3a plays a significant role in triggering the inflammatory response, helping to eliminate pathogens, while C3b acts as an opsonin, enhancing phagocytosis by marking microbes for destruction. Additionally, C3b can interact with other complement proteins to form membrane attack complexes, which create pores in the membranes of gram-negative bacteria, leading to their lysis.

This overview of the complement system highlights its importance in both innate and adaptive immunity, illustrating how different pathways converge to enhance the body's defense mechanisms against infections.

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.

Answer: (E)

Which complement molecule is an attractant for neutrophils?

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

Answer: (B)

The key molecule upon which all complement pathways converge is

• A. C1.
• B. C2.
• C. C3.
• D. C6.

Answer: (C)

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.

Answer: (C)

Which of the following are most susceptible to complement lysis?

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

Answer: (B)

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 ultimately lead to the formation of C3 convertase, which cleaves the inactive protein C3 into C3a and C3b, playing crucial roles in immune responses.

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

C3 convertase is a crucial enzyme in the complement system that cleaves the inactive protein C3 into two fragments: C3a and C3b. C3a plays a role in triggering the inflammatory response, which helps to eliminate microbes. C3b acts as an opsonin, enhancing opsonization and improving the effectiveness of phagocytosis by phagocytes like macrophages. Additionally, C3b interacts with other complement proteins to form membrane attack complexes, leading to the lysis of pathogen cells. Thus, C3 convertase is essential for amplifying the immune response against invading microbes.

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

The classical pathway of the complement system complements adaptive immunity by involving antibodies from the adaptive immune system. When these antibodies bind to antigens on the surface of microbes, they trigger a cascade of events that lead to the formation of C3 convertase. This interaction between antibodies (adaptive immunity) and the complement system (innate immunity) enhances the overall immune response. The classical pathway exemplifies the crosstalk between innate and adaptive immunity, making the immune system more effective in combating pathogens.

What are the effects of C3a and C3b in the complement system?

C3a and C3b, produced by the cleavage of C3 by C3 convertase, have distinct roles in the complement system. C3a is involved in triggering the inflammatory response, which helps to recruit immune cells to the site of infection and eliminate microbes. C3b acts as an opsonin, enhancing opsonization and making it easier for phagocytes like macrophages to engulf and destroy pathogens. Additionally, C3b contributes to the formation of membrane attack complexes, which create pores in the membranes of pathogens, leading to their lysis and death.

What is the significance of the membrane attack complex in the complement system?

The membrane attack complex (MAC) is a crucial component of the complement system that leads to the lysis of pathogen cells. Formed by the interaction of C3b with other complement proteins, the MAC creates pores in the membranes of gram-negative bacteria and other pathogens. These pores disrupt the integrity of the microbial cell membrane, causing cell lysis and death. The formation of the MAC is an essential mechanism by which the complement system directly eliminates invading microbes, thereby protecting the host from infections.