Virulence factors are specialized bacterial components that promote evasion of host immune defenses, enabling pathogens to survive, replicate, and cause disease.
Key Virulence Factors
| Virulence Factor | Mechanism of Action | Examples |
|---|---|---|
| Capsular polysaccharide | It is a hydrophilic, highly charged structure that prevents phagocytosis and complement lysis. | Streptococcus pneumoniae, Klebsiella pneumoniae, Haemophilus influenzae |
| Protein A | Binds the Fc portion of IgG → blocks opsonization and phagocytosis. | Staphylococcus aureus |
| IgA protease | Cleaves IgA → allows colonization of mucosal surfaces. | Streptococcus pneumoniae, Haemophilus influenzae type b, Neisseria (SHiN) |
| M protein | Inhibits phagocytosis. Molecular mimicry with cardiac myosin may trigger autoimmunity. | Group A Streptococcus (S. pyogenes) |
Learning Objective (USMLE Step 1): Recognize major bacterial virulence factors, their mechanisms of immune evasion, and their association with specific pathogens (e.g., S. aureus → Protein A, S. pyogenes → M protein).
