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Natural Defense Mechanisms
Immunology Unit
Dept. of Pathology
College of Medicine
KSU
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2
Reference
Kuby Immunology 7th Edition
2013Chapter 4 Pages 105-110
Chapter 5 Pages 141-176
Chapter 6 Pages 187-200 & page 213
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Objectives
o + To know First (non-specific immunity) and
second (adaptive immunity) lines of defense
o + To understand the Complement system, its
activation and how it involves in pathogen
killing.
o + To recognize the importance of
accumulation of inflammatory cells for
clearance of infection
o + To know the role of cytokines as mediators
which regulate inflammation 3
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Bacteria e.g.Tubercule bacillusStaphylococci
Fungi e.g.Candida albicans
Viruses e.g.
Influenza
Polio
Parasites e.g.Tapeworms
Malaria
The main function of the immune system is to
protect from infections:
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First and the second lines of defense:
First Line of Defense Second Line of Defense
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First line of defense :
◼ Natural (Innate) Immunity
◼ Physical (skin/ mucous membranes )
◼ Mechanical (Coughing, sneezing, vomiting, action of cilia in trachea)
◼ Biochemical barriers (antimicrobial peptides, lung secretions, mucus, saliva, tears)
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Physical and mechanical barriers
◼ Skin, impermeable to microbes.
◼ Mucous membranes lining the gastrointestinal,
genitourinary and respiratory tracts.
◼ Other protective mechanisms:
Shedding of outer skin layers.
Coughing and sneezing.
Flushing of urine.
Vomiting.
Mucus and cilia in respiratory tract.
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Biochemical barriers
◼ Body secretions contain anti-bacterial substances
e.g. saliva, tears and sweat.
◼ Antimicrobial peptides (e.g., defensins, hepcidins)
◼ Normal bacterial flora.
(Compete with pathogenic bacteria for nutrients)
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◼ Inflammation:◼ Inflammation is the first response of the
immune system to infection or irritation.
◼ It consist of a series of vascular & cellular changes that occur in response to various
stimuli
e.g. infections, injury, radiation etc.
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As bacteria possess an array of pro-inflammatory
molecules:
e.g. Lipopolysaccharides (LPS)
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Microbial infections initiate inflammation
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Inflammation
Goals
◼ Prevent and limit infection and further damage
◼ Interact with adaptive immune system
◼ - For example Monocytes / Macrophages
serve as a link between the adaptive and
innate immunity by antigen presentation
◼ Prepare the area of injury for healing
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The Complement system
◼ Consists of a group of serum proteins initially present in inactive form
◼ Activation occurs in cascade ( one component activating another) after enzymatic clivage. Once components become activated they produce important biological effects that initiate inflammation.
◼ This system plays an important role in linking Innate & Adaptive immunity
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The complement system:
3 Pathways of activation :
* Classical. (Requires antigen-antibody binding)
(C1,C4,C2,C3,C5,C6,C7,C8,C9)
* Lectin. (Activated by mannan binding proteins
binding mannose groups of bacterial carbohydrates)
- (MASPs, C4,C2,C3,C5,C6,C7,C8,C9)
* Alternative.(Activated by bacterial products)
- (C3, B, D, C5,C6,C7,C8,C9)
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C3b
C3c
Enzymatic cleavage
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C5 convertases
C5 C5a + C5b
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Membrane Attack Complex formation
C5-activation
C3bC4b
b
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Components of the Membrane Attack Complex
C6
C9
C7
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Membrane Attack Complex components Assembly
b
C6
C7
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Membrane Attack Complex formation :insertion of lytic complex into cell membrane
b
C6
C7
C9
C9
C9
C9C
9C9 C
9C9
C9
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Biological effects of complement activation
1. Anaphylatoxin (C3a, C5a) (recruit more cells to help
out !!!)
- Induce histamine and cytokines release from mast
cells
- blood supply by binding to endothelial cells
- Attraction of additional inflammatory cell to the site
of activation
2. Opsonization: (opsonin, C3b and C4b ) ( helps to eat
microbes !!!!)
- Coating of bacteria enhances phagocytosis
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3. Cause direct cell lysis
- Destruction of bacteria. (blasts holes in
bacteria making them leak so die !!!)
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Process of chemotaxis:
Rolling on vessel wall. Adhesion (attach) Pass through. :
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Types of Cells attracted to site of infection
that mediate inflammation :
Monocytes :◼ Become Macrophages when they leave the blood and
enter the tissues.
Neutrophils: (Phagocytic cells)
Eosinophils: (Allergy and Parasitic infections)
Natural Killer (NK) cells: (Kill tumor cells and
virus infected cells)
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Phagocytic cells (neutrophils & macrophages) at
site of infection start the process of phagocytosis
which is the process by which a cell engulf a solid
particle such as bacteria to form internal vesicle
known as phagosome
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Macrophage attacking E. coli
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Phagocytosis
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Cytokines
Soluble molecules, produced by different cells, that
control cell functions e.g. differentiation, proliferation
activation or inhibition.
e.g. Interleukins
◼ Produced primarily by macrophages and
lymphocytes in response to a pathogen.
◼ Many types/ IL-1, IL-2, IL-3…....
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Cytokines
Interferons:
◼ Protects against viral infections
◼ Produced and released by virally infected cells in
response to viral infections.
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Cytokines
Tumor necrosis factor (TNF)
◼ Secreted by macrophages.
- Induces fever by acting as an endogenous pyrogen (a
substance released from inside the body that produces
fever)
- Increases synthesis of inflammatory serum proteins/ C
reactive protein
- - Increase expression of adhesion molecules on
endothelial cells and vascular permeability
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Take home message
1. Non-specific (innate immunity) acts as a
first line of defense against invading
pathogens
2. Innate immunity is an important initial step
for generation of adaptive immune response
3. Inflammation is vital for controlling
infection and limiting tissue damage
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