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==Overview==
The immune mechanisms that defend against bacterial infection relate to the type of cell wall (see structure) and recognition of common bacterial components.
'''Evading immune defences'''[[Image:S aureus.jpg|thumb|right|100px|'''S. aureus''' Brian0918 2006, WikiMedia Commons]]
==Innate mechanisms==
===Lipopolysaccharide recognition===
LPS is a component of some Gram-negative bacterial cell walls and its recognition is thought to be an ancient mechanism that evolved before the acquired immune system. When released by bacteria, LPS can bind to soluble CD14, which causes the release of TNF-alpha and IL-1 (both lead to systemic phagocyte activation), or to lipoprotein particles, which neutralize it.
===Complement===
Some bacteria, particularly those with an outer lipid bilayer (i.e. Gram-negative), are susceptible to [[Complement]] activated via the alternative pathway (the lytic complex: C5b-9). The release of C3a and C5a lead to histamine release, and attracts and activates [[Neutrophils|neutrophils]]. Components of C3 aid opsonisation of the bacteria.
==Phagocytosis==
Most bacteria are killed this way. [[Complement products]], bacterial components (e.g. f-Met-Leu-Phe) and locally released cytokines are chemotactic for phagocytes. The binding of the phagocyte can be mediated by the following;
***Lectins on the bacteria, e.g. mannose-binding lectin of E. coli.
***Lectins on the phagocyte, e.g. complement receptors such as CR3.
***[[Complement]] deposited on the organism, both classic and alternative pathways.
***Fc receptors on phagocyte, bind to the antibody on bacteria.
The activation of [[Macrophages|macrophages]] occurs after exposure to microbial products or T cell-derived lymphokines. Lymphokines released during T-cell activation are often required for full activation, such as IFN-γ.
==Immunopathology==
The release of endotoxins, typically LPS by Gram-negative bacteria, can cause the excessive production of [[Cytokines|cytokines]] that lead to 'endotoxin shock.' This can lead to fever, circulatory collapse and haemorrhagic necrosis, and eventually multiple organ failure and potentially death. Gram-positive bacteria can cause a similar reaction. The Koch phenomenon is the necrotic response observed in [[Mycobacterium tuberculosis|M. tuberculosis]] infections and is thought to be partly due to the release of [[Cytokines|cytokines]].
==Evading immune defences==[[Image:S aureus.jpg|thumb|right|100px|'''S. aureus''' Brian0918 2006, WikiMedia Commons]]
Bacteria can avoid the [[Complement|complement]] reponse in the following ways;
**Proteins can be expressed on the surface that divert the lytic complex from the cell membrane
**Proteins can be expressed on the surface that divert the lytic complex from the cell membrane
**The outer membrane can resist the lytic complex
**The outer membrane can resist the lytic complex
<big>'''Also see [[Innate Immunity to Bacteria]] and [[Adaptive Immunity to Bacteria]]'''</big>
<big>'''Also see [[Innate Immunity to Bacteria]] and [[Adaptive Immunity to Bacteria]]'''</big>
[[Category:Bacteria]]
[[Category:To Do - AimeeHicks]]