Complement activation

Definition

Complement activation is a crucial part of the immune response, where a series of proteins in the blood are activated in a cascade manner to help clear pathogens from an organism. This process enhances the ability of antibodies and phagocytic cells to clear microbes and promote inflammation, ultimately leading to pathogen destruction. It is closely linked to antibody action, as antibodies can trigger complement activation when they bind to antigens on the surface of pathogens.

5 Must Know Facts For Your Next Test

1. Complement activation can occur via three main pathways: the classical pathway, the alternative pathway, and the lectin pathway, each initiated by different triggers.
2. The classical pathway is primarily activated by antibodies bound to antigens, showcasing the link between antibody function and complement activation.
3. Once activated, complement proteins can lead to the formation of the membrane attack complex (MAC), which creates pores in pathogen membranes, leading to cell lysis.
4. Complement activation also generates small peptides that act as anaphylatoxins, promoting inflammation and attracting immune cells to sites of infection.
5. Deficiencies or dysregulation in the complement system can lead to increased susceptibility to infections or contribute to autoimmune diseases.

Review Questions

• How does complement activation enhance the effectiveness of antibodies in the immune response?
  • Complement activation enhances antibody effectiveness by creating a series of reactions that amplify the immune response. When antibodies bind to pathogens, they trigger the classical pathway of complement activation. This not only promotes opsonization, making it easier for phagocytic cells to recognize and engulf pathogens, but also leads to the formation of the membrane attack complex, directly lysing target cells. Together, these actions increase the efficiency of antibody-mediated immunity.
• Compare and contrast the three pathways of complement activation and their significance in immune defense.
  • The three pathways of complement activation—the classical pathway, alternative pathway, and lectin pathway—differ primarily in how they are triggered. The classical pathway relies on antibody-antigen complexes, while the alternative pathway is initiated by pathogen surfaces directly, and the lectin pathway is activated by mannose-binding lectin binding to carbohydrates on pathogens. All pathways converge on a common endpoint that leads to pathogen opsonization and lysis, highlighting their significance in providing multiple layers of immune defense against infections.
• Evaluate the role of complement activation in both enhancing protective immunity and contributing to pathological conditions.
  • Complement activation plays a dual role in immunity: it enhances protective mechanisms against pathogens while also having the potential to contribute to pathological conditions. On one hand, it effectively aids in clearing infections through opsonization and cell lysis. On the other hand, excessive or uncontrolled complement activation can lead to tissue damage and contribute to autoimmune diseases, where the immune system mistakenly attacks the body's own cells. Understanding this balance is crucial for developing therapies targeting complement-related disorders.

Related terms

Complement System: A group of proteins in the blood that work together to enhance immune responses through opsonization, inflammation, and lysis of pathogens.

Opsonization:

The process by which pathogens are marked for destruction by phagocytes through the binding of antibodies or complement proteins.

Cytokines:

Small proteins released by cells that have a specific effect on the interactions and communications between cells, often playing a key role in regulating immune responses.