Lecture Notes: The Complement System

Introduction

• The complement system involves a series of proteins in the blood, crucial for immune responses.
• These proteins are produced in the liver and released into the bloodstream.
• Known as complement proteins, they are labeled from C1 to C9.

Pathways of the Complement System

• Classical Pathway

  • Triggered by the binding of an antibody to an antigen on a pathogen.
  • Activation process involves:
    1. C1 Activation: Binds to the antigen-antibody complex.
    2. C2 and C4 Splitting: Splits into fragments.
    3. C3 Convertase Formation: Combines fragments of C2 and C4 to form C3 convertase.
    4. C3 Splitting: C3 convertase splits C3 into C3a and C3b.
    5. C3a Function: Acts as a chemical attractant for phagocytes.
    6. C3b Function: Tags the pathogen for destruction; facilitates C5 splitting.
    7. Membrane Attack Complex: Involves C5b, C6, C7, C8, and C9 forming a complex that creates a hole in the pathogen membrane.

• Alternative Pathway

  • Does not require antigen-antibody complex; starts directly at C3.
  • Steps involved:
    1. C3 Activation: Binds to the pathogen itself.
    2. Interaction with Factors P, B, and D: Activates C3 splitting.
    3. Continuation: Follows same pathway as classical once C3 is split.

Importance of Pathways

• Classical Pathway: Part of the specific immune response, requiring an antigen-antibody complex.
• Alternative Pathway: Non-specific, directly binding to pathogens initiating the cascade.
• Outcomes:
  • Recruitment of phagocytes.
  • Pathogen tagging and destruction.
  • Formation of membrane attack complex leading to pathogen lysis.

Conclusion

• Complement system is a sophisticated defense mechanism.
• Highlights the body's optimized processes for protecting against pathogens.
• Next topic: Role of T-cells in adaptive immunity.