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What are three specific factors of the immune system that help to clear the lower lung of infectious agents?
Source  Cowan, M.K. (2012) Microbiology: A Systems Approach (3rd ed.) New York, NY: McGraw-Hill
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The three specific factors of the immune system that help to clear the lower lung of infectious agents include the pulmonary clearance, generation of antibodies against the infectious agent and the greater surface area of the lower lung region help to clear the lower lung of infectious agents.
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The respiratory tract of human divided into upper, airways , lower respiratory tracts, these contains a layer of cells called alveolar cells,these lined up inner tract by cells called pneumocystes which follows defense mechanism to protect parenchymal cells from invasion with micro organisms.

Mostly lower lung infections are pneumonia, bronchitis and whooping cough. pneumonia is caused by bacterial pathogen called Streptococcus pneumoniae

The alveolar cells have defensive mechanism by the presence of

Alveolar macrophages
Complement components
Alveolar linings contains various n surfactants, phospholipid, neutral lipids, IgG, IgE, IgA, Factor B and other agents
B cells, T cells and null cells that can elicit localized immune response to infections

Lymphoid tissue associated with the lungs.

When bacteria enters into the cells the three important factors invovled in host defense mechanism :-

Neutrophils induced defense and lung damage is associated with lower lung infections, these pathogens initially interact with cells in alveoli cells and macrophages induces secretion of cytokines and neutrophils I.e. chemoattractants
This cytokines upregulated expression of cell adhesion molecules on capillary endothelia and mediates transmigration of neutrophils into alveolar spaces based on chemotactic gradients
These cells produces proteases, reactive oxygen species ROS and Reactive nitrogen species RNS induces necrotic cell death gradually extensive lung injury and RBCs are lysed.
Neutrophils, macrophages and dendritic cells of host innate immune response are activated to clear lower lung infections.

In this context pathogens develop specific mechanism to protect from host defence of natural immune response by

Capsule production (S.pneumoniae)
Toxin production
Large size difficult to phagocytosed
Replication inside cells intracellular parasites
Mimics pathogen as host cell (protein A of S aureus)
Inhibition of phagolysosome in macrophages

The diagram illustrated the mechanism of lower lung disease
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