Lecture Notes: The Complement System

Introduction

• Every living being needs defense mechanisms against invaders.
• Multicellular life evolved sophisticated defense networks, including humans.
• The complement system is a crucial but lesser-known defense.

Complement System Overview

• Evolved over 700 million years ago.
• Consists of over 30 different proteins.
• Proteins work together to stop intruders with around 15 quintillion proteins saturating body fluids.
• Functions without direction, guided by chemistry.
• Critical part of the immune system, but potentially dangerous if uncontrolled.

Functions of the Complement System

1. Cripples Enemies
2. Activates the Immune System
3. Destroys Invaders

Activation Mechanism

• Proteins are passive until activated, altering their shape.
• Shape change allows interaction with other proteins, starting activation cascades.
• Similar to matches igniting a fire.

Action Process

1. Initial Activation
   • Starts with protein C3, which breaks into C3a and C3b.
   • C3b acts as a seeker missile for bacteria, fungi, and viruses, anchoring to invader surfaces.
2. Amplification Loop
   • C3b recruits other proteins, forming C3 Convertase, and activates more C3.
   • Thousands of proteins can coat bacteria, crippling them.
3. Signal for Reinforcements
   • C3a proteins act as distress beacons, drawing immune cells to the infection site.
   • Increases immune response aggression through protein signals.

Destruction of Invaders

• Phagocytes arrive, and complement acts as glue for easier bacterial capture.
• Formation of a membrane attack complex by newly recruited proteins.
• Drills holes into bacterial membranes, causing them to bleed to death.

Effectiveness Against Viruses

• Viruses are vulnerable outside cells.
• Complement system intercepts, cripples, and guides immune system to destroy them.
• Without complement, viral infections would be more lethal.

Limitations and Adaptations

• Both sides of a biological war adapt.
• Some viruses produce proteins to inhibit complement activation.
• Some bacteria mask themselves by capturing molecules to evade complement.

Conclusion

• Complement system is vital but only one part of the immune system.
• Demonstrates how simple components can perform complex tasks collaboratively.