Antimicrobial Defense: Complement System and Interferons
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VII. Antimicrobial Substances
A. The Complement System
The complement system is a vital component of the nonspecific defense system. It consists of a group of serum proteins that activate one another in a cascade to destroy invading microorganisms.
1. Evasion Mechanisms
Some bacteria evade destruction by the complement system through:
- Capsules
- Surface lipid–carbohydrate complexes
- Enzymatic destruction of C5a
2. Functional Groups
Complement proteins form a membrane attack complex that perforates the plasma membrane, causing cell lysis. They are subdivided into three functional groups:
- C1: Recognition
- C4, C2, C3: Activation
- C5–C9: Attack (complement fixation)
3. Complement System Pathways
- Classic Pathway: Antibody binding to pathogen antigens activates