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Anti-Inflammatory Responses• Complement regulatory proteins: e.g. C1 inhibitor,
• Production of anti-Ig proteases• Destruction of phagocyte• Inhibition of chemotaxis• Inhibition of phagocytosis• Inhibition of phagolysosome
fusion• Resistance to lysosomal
enzymes• Superantigens
Mechanisms of Immune Evasion I: Camouflage
• Capsule formation• S aureus protein A• Sialic acid• LPS O protein• S aureus coagulase• M bacterium granuloma formation
Mechanisms of Immune Evasion III: Anti-Phagocytosis
• Inhibit opsonization (S aureus protein A)• Inhibit chemotaxis• Kill phagocyte (S aureus streptolysin)• Inhibit phagocytosis (S pneumoniae capsule, S pyogenes M protein)• Inhibit lysosomal fusion (M. tuberculosis)• Escape lysosome and grow in cytoplasm (Mycobacteria, Salmonella, S.
aureus)• Block activation by IFN (Mycobacteria)
• Viral envelope glycoproteins
• LPS
Mechanisms of Immune Evasion II: Proteases
• Inhibit opsonization (N gonorrhoeae IgA protease)• Inhibit chemotaxis• Kill phagocyte (S aureus streptolysin)• Inhibit phagocytosis (S pneumoniae capsule, S pyogenes M protein)• Inhibit lysosomal fusion (M. tuberculosis)• Escape lysosome and grow in cytoplasm (Mycobacteria, Salmonella, S.
aureus)• Block activation by IFN (Mycobacteria)
• Viral envelope glycoproteins
• LPS
Viral Mechanisms of Immune Evasion I
• Humoral Response
Latency e.g. HSV, retroviruses Syncytia formation e.g. HSV, VZV, HIV Antigenic variation e.g. HIV Blocking antigen e.g. HBV e Ag Complement decay e.g. HSV
Viral Mechanisms of Immune Evasion II.
• Interferon HBV blocks transcription of IFN EBV synthesizes BRC1, an analogue of
IL-10. Adenovirus RNA - double stranded
duplex blocks interferon antiviral action; early protein binds cl I heavy chain preventing upregulated expression
Viral Mechanisms of Immune Evasion III
• Immune Cell Function CTL cytolysis e.g. HSV TH depletion e.g. HIV Immunosuppression e.g. measles,
EBV
Viral Mechanisms of Immune Evasion IV.
• Antigen Presentation Inhibition of Cl I MHC expression e.g.
Adenovirus, CMV Inactivating peptides e.g. HBV
• Inhibition of Inflammation Blocking of inflammatory cytokines e.g.
Poxviruses, adenovirus.
Infection and Pathogenesis
Colonization (Benign or asymptomatic)
Infection Disease (Pathogenesis)
Clinical or Subclinical
Requisites for Successful Growth
• Attachment
• Nutrition
• Survival from host defence
• Transmission
Virulence Factors
• Factors which promote infection and which contribute to disease
• Studied with mutants• Are multifactorial• Consist of:
Factors promoting colonization and invasion Factors which are pathogenic
Bacterial Virulence Factors I: Colonization
• Adherence: Capsules, Pili, adhesins
• Penetration: e.g. invasins
• Host gene modification.
Capsules• Present in some gram negative and positive bacteria.• May be composed of protein or polysaccharide
layers.• Is poorly antigenic and anti-phagocytic• Can act as a barrier to toxic hydrophobic molecules
such as detergents.• Can promote adherence to other bacteria or cell
surfaces
Pili (Fimbriae)
• Composed of subunits of pilin.• Promote adherence to other bacteria or
host.• Synonyms: adhesins, lectins, evasins,
aggressins.• Fragile, often replaced.
Bacterial Pathogenesis
• Toxic byproducts of bacterial growthe.g. acids, gas, proteases
• Toxins Endotoxins e.g. LPS Exotoxins
• Immunopathogenesis e.g. Chlamydia, treponemes (syphilis), Borrelia (Lyme disease)
Endotoxins: Lipopolysaccharide• Fever• Leukopenia, followed by