C5b

Definition

C5b is a component of the complement system, a complex network of proteins that plays a crucial role in the body's immune defense against pathogens. C5b is a central part of the terminal complement pathway, which leads to the formation of the membrane attack complex (MAC) and ultimately cell lysis.

5 Must Know Facts For Your Next Test

1. C5b is generated when the complement protein C5 is cleaved by the C5 convertase enzyme, exposing a reactive thioester group that allows C5b to bind to the target cell membrane.
2. The binding of C5b to the cell membrane initiates the assembly of the membrane attack complex, which is composed of C5b, C6, C7, C8, and multiple C9 proteins.
3. The formation of the MAC creates a pore in the target cell membrane, leading to the influx of water and ions, osmotic lysis, and ultimately, cell death.
4. The terminal complement pathway, including the formation of C5b, is an important defense mechanism against Gram-negative bacteria, as the MAC can directly lyse their cell membranes.
5. Regulation of the complement system, including the C5b-mediated formation of the MAC, is crucial to prevent excessive or inappropriate activation, which can lead to tissue damage and autoimmune disorders.

Review Questions

• Explain the role of C5b in the complement system's terminal pathway and its importance in immune defense.
  • C5b is a central component of the terminal complement pathway, which leads to the formation of the membrane attack complex (MAC) and the subsequent lysis of target cells. When the complement protein C5 is cleaved, C5b is generated and binds to the target cell membrane, initiating the assembly of the MAC. The MAC creates a pore in the cell membrane, causing an influx of water and ions, leading to osmotic lysis and cell death. This process is particularly important in the immune defense against Gram-negative bacteria, as the MAC can directly lyse their cell membranes. Regulation of the C5b-mediated formation of the MAC is crucial to prevent excessive or inappropriate activation, which can lead to tissue damage and autoimmune disorders.
• Describe the relationship between C5b and the membrane attack complex (MAC), and explain how their interaction leads to cell lysis.
  • The binding of C5b to the target cell membrane initiates the assembly of the membrane attack complex (MAC), which is composed of C5b, C6, C7, C8, and multiple C9 proteins. C5b acts as the anchor, allowing the other MAC components to sequentially bind and form the pore-like structure in the cell membrane. The formation of the MAC creates a channel that allows the influx of water and ions into the target cell, leading to osmotic lysis and cell death. This process is a critical step in the terminal complement pathway, as the MAC-mediated lysis of target cells is a key mechanism of immune defense, particularly against Gram-negative bacteria.
• Analyze the importance of regulating the C5b-mediated formation of the membrane attack complex and its implications for immune system function and disease.
  • Proper regulation of the C5b-mediated formation of the membrane attack complex (MAC) is crucial for maintaining immune system homeostasis and preventing tissue damage. Excessive or inappropriate activation of the complement system, including the terminal pathway involving C5b, can lead to the uncontrolled formation of the MAC and the lysis of host cells, contributing to the development of autoimmune disorders and other inflammatory diseases. Conversely, impaired regulation of the complement system and the C5b-MAC pathway can increase susceptibility to infections, as the immune system's ability to effectively eliminate pathogens is compromised. Understanding the delicate balance required in the C5b-MAC axis is essential for developing targeted therapies to modulate the complement system and address both autoimmune and infectious disease conditions.

Related terms

Complement System:

The complement system is a group of proteins that work together to identify, opsonize, and destroy pathogens, as well as promote inflammation and facilitate the clearance of immune complexes and damaged cells.

Membrane Attack Complex (MAC): The membrane attack complex is the final step in the complement cascade, where C5b, C6, C7, C8, and multiple C9 proteins assemble to form a pore in the target cell membrane, leading to cell lysis and death.

Terminal Complement Pathway: The terminal complement pathway is the final stage of the complement system, where the activation of C5 leads to the formation of the membrane attack complex and the subsequent lysis of the target cell.