Complement activation

Definition

Complement activation is a crucial part of the immune system that enhances the ability of antibodies and phagocytic cells to clear pathogens from an organism. It involves a series of proteins that are activated in a cascade, leading to opsonization, inflammation, and cell lysis. This process is vital in the context of viral infections, where it helps eliminate viruses and infected cells while also playing a role in the immunopathology that can arise during such infections.

5 Must Know Facts For Your Next Test

1. Complement activation can occur through three pathways: the classical pathway, the alternative pathway, and the lectin pathway, each triggered by different stimuli.
2. In viral infections, complement activation can help neutralize viruses directly or enhance their clearance by phagocytes.
3. While complement activation is protective, it can also contribute to tissue damage and inflammation in cases of excessive or inappropriate activation.
4. The balance between complement activation and regulation is essential; dysregulation can lead to autoimmune diseases or chronic inflammatory conditions.
5. Complement proteins are synthesized primarily in the liver and circulate in an inactive form until they are activated by pathogens or immune complexes.

Review Questions

• How does complement activation enhance the innate immune response during viral infections?
  • Complement activation enhances the innate immune response by opsonizing viruses, making them more recognizable to phagocytes for elimination. It also generates anaphylatoxins that promote inflammation and recruit additional immune cells to the site of infection. These combined actions improve the overall efficiency of the immune system in controlling viral infections.
• Discuss the potential immunopathological consequences of complement activation during viral infections.
  • While complement activation plays a protective role against viruses, it can lead to immunopathological consequences when activated excessively. This may result in tissue damage due to inflammation and collateral damage from the recruitment of immune cells. Additionally, excessive complement activity can contribute to conditions like acute respiratory distress syndrome (ARDS) seen in severe viral infections, illustrating the need for balance in this immune response.
• Evaluate the mechanisms through which complement activation can be regulated to prevent tissue damage while maintaining effective antiviral responses.
  • Complement activation is tightly regulated by several control proteins that prevent over-activation and protect host tissues from damage. These regulatory mechanisms include factors like C1 inhibitor and factor H, which inhibit various stages of complement cascade activation. By maintaining this balance, the immune system can effectively clear viral infections without causing excessive inflammation or tissue injury, thereby ensuring a robust yet controlled immune response.

Related terms

Opsonization: The process by which pathogens are marked for ingestion and destruction by phagocytes, facilitated by the binding of complement proteins.

Anaphylatoxins: Small peptide fragments generated during complement activation that promote inflammation and recruit immune cells to sites of infection.

Membrane Attack Complex (MAC): A structure formed by the terminal complement components that creates pores in the membranes of pathogens, leading to cell lysis.