Membrane attack complex

Definition

The membrane attack complex (MAC) is a structure formed by the complement system, specifically the terminal complement proteins, which creates pores in the membranes of target cells, leading to cell lysis and death. This mechanism is a critical part of the innate immune response, serving to eliminate pathogens such as bacteria and some viruses by compromising their cell membranes. The formation of the MAC is a result of a cascade reaction initiated by immune recognition, emphasizing its role in enhancing the body's defense against infection.

5 Must Know Facts For Your Next Test

1. The MAC is primarily composed of complement proteins C5b, C6, C7, C8, and multiple C9 molecules that assemble on the target cell's surface.
2. Once formed, the MAC creates large pores in the target cell membrane, disrupting its integrity and causing water and ions to flow into the cell, leading to osmotic lysis.
3. The formation of the MAC can be triggered by various pathways of the complement system, including the classical, lectin, and alternative pathways.
4. In addition to directly lysing pathogens, the MAC can also contribute to inflammation by attracting immune cells to the site of infection.
5. Some pathogens have evolved mechanisms to evade destruction by the MAC, including alterations in their membrane composition or expression of proteins that inhibit complement activation.

Review Questions

• How does the formation of the membrane attack complex contribute to the innate immune response?
  • The formation of the membrane attack complex (MAC) plays a vital role in the innate immune response by directly targeting and lysing pathogenic cells. When activated through various complement pathways, the MAC assembles on the surface of pathogens like bacteria, creating pores that compromise their membranes. This leads to cell lysis and death, effectively reducing the pathogen load and contributing to overall immune defense.
• Discuss how different pathways lead to the activation of the membrane attack complex and their significance in immune defense.
  • The membrane attack complex can be activated via three main pathways: classical, lectin, and alternative. The classical pathway is triggered by antibodies bound to antigens, while the lectin pathway relies on mannose-binding lectin recognizing sugars on pathogens. The alternative pathway can be spontaneously activated on pathogen surfaces. Each pathway converges at a point where C5 convertase is formed, ultimately leading to MAC formation. This diversity allows for a robust immune response against various types of pathogens.
• Evaluate the impact of pathogens' evasion strategies on the effectiveness of the membrane attack complex within innate immunity.
  • Pathogens have developed various evasion strategies that can significantly impact the effectiveness of the membrane attack complex (MAC). Some bacteria modify their surface structures to resist complement deposition or actively produce proteins that inhibit complement activation. These adaptations can prevent MAC formation or reduce its efficiency in lysing target cells. Understanding these evasion mechanisms is crucial for developing better therapeutic strategies and improving our overall approach to fighting infections.

Related terms

Complement system:

A group of proteins in blood plasma that work together to enhance the ability of antibodies and phagocytic cells to clear pathogens from an organism.

Cytolysis: The process in which a cell's membrane is compromised, leading to its rupture and death, often as a result of the action of the MAC.

Innate immunity:

The first line of defense against pathogens, consisting of physical barriers, immune cells, and soluble factors that respond rapidly to infection.