Complement System Biology

Complement System Activation

• The complement system is activated by the presence of pathogens, leading to the destruction of the pathogen through the formation of the Membrane Attack Complex.
• C3b, a product of the complement system, acts as an opsonin, marking pathogens for destruction by immune cells.

Chemotaxis and Cell Activation

• C5a, a complement system component, attracts immune cells, including neutrophils, monocytes, and eosinophils, to the site of infection.
• C3a, C4a, and C5a activate mast cell degranulation, releasing histamine and other mediators.
• C3a and C5a also activate eosinophil degranulation, contributing to the inflammatory response.

Innate Immune System Activation

• Macrophages recognize pathogens and activate the innate immune system, triggering a rapid response to infection.
• Dendritic cells, upon recognizing pathogens, pick up antigens and migrate to activate the specific immune system, leading to a targeted response to infection.

Macrophage Activation

• Macrophages recognise pathogens and initiate phagocytosis
• Release cytokines and interferons as immune responses
• Interferons inhibit viral entry and replication within cells

Cytokine Functions

• Cytokines recruit help by activating monocytes to become macrophages
• Cytokines initiate an inflammatory response, leading to:
  • Vasodilation
  • Increased vascular permeability
  • Localised endothelial cell activation
• Cytokines activate other systems, including:
  • Mast-cell degranulation
  • Clotting system activation
  • Kinin system

Inflammatory Response and Acute Phase Response

• Inflammation leads to an acute phase response, characterised by:
  • Macrophages and neutrophils secreting more cytokines, including interleukins
• Interleukins have various functions, including:
  • IL-1: causing fever, lethargy, and anorexia
  • IL-6: stimulating the liver to produce acute phase proteins (opsonins)
  • IL-8: recruiting and activating neutrophils
  • IL-2 and IL-12: activating natural killer cells

Natural Killer Cells

• Natural killer cells are recruited to tissues and:
  • Kill virally infected cells (and tumour cells)
  • Produce interferon-gamma (IFN-γ) that further stimulates macrophages

Learn about the complement system, a part of the innate immune system that helps eliminate pathogens. Understand how it activates and destroys pathogens, and its role in immune response.