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Physical and Chemical
Barriers to Infection
Week 12
Physical Barriers
Skin
 Mucus (snot!)
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Elaine Chen’s animation
Chemical barriers
Interferon (see later)
 Cytokines
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Group of proteins secreted by immune cells to recruit and
activate other immune cells (activates either humoral or
cellular response)
Complement proteins (see later)
Lymphatic system
Distribution of
lymphoid organs and
tissues which make up
the immune system
Lymph nodes
Lymphatic system
Blood cells
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Human blood showing
different kinds of blood
cells.
Various white blood cells
(phagocytes) have densely
staining nuclei.
White blood cells
White blood cells (phagocytes)
Neutrophil cells that have
ingested bacteria.
 The bacteria appear as the
smaller purple rod shapes inside
the cells
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Radioactively labelled
macrophages and a few
lymphocytes
Non-specific vs. Specific responses
Non-specific response
Pathogen Invades Tissue
Specific defences
(see next section)
Non-specific defences
Barriers
to entry
Physiological
mechanisms
Fever
Chemical
mechanisms
Complement
proteins
Interferon
Phagocytes
and natural
killer (NK)
cells
Inflammation
Basophils
Mast cells and
Platelets
Histamine,
Phagocytosis
First line of defence
The first line of defence against infection is the body surface which
acts as a barrier (physical/chemical barriers – already discussed)
Chemicals on the body surface also inhibit infective organisms
Second line of defence
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Includes:
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Fever
Interferon/Cytokines
Complement proteins
Phagocytes (white blood cells)
Natural killer (NK) cells
Inflammation
Blood clotting
Fever
Fever
Interferon/ Cytokines

Interferon/cytokines
Complement proteins
Complement proteins
lyse many bacterial
species.
 This attracts
phagocytes to the site
of infection.
 Bacteria which have
been coated by other
complement proteins
are readily ingested
by the phagocytes.

Complement proteins
Complement proteins
Phagocytosis
Stages of phagocytosis: a neutrophil ingesting a bacterium
Macrophage animation
Natural Killer (NK) cells
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3 animations
Inflammation
Inflammation occurs if bacteria enter a cut
a)
b)
c)
d)
Injury to an otherwise healthy skin
Vasodilation and increased permeability
Phagocyte migration from capillaries to cut area
Phagocytosis of bacteria and other debris by macrophages
Inflammation
Blood vessels
dilate
Pathogens
enter
tissues
Mast cells
Basophils
Platelets
Complement
proteins attract
phagocytes
Produce
Histamine
and other
substances
Increased
blood flow to
the region
Capillaries
become
permeable
and leaky
Phagocytes
move to the
area
Redness
Heat
Edema
Pus
Increased
Phagocytes
Blood Clotting
Red blood cells
trapped in protein
fibres
Some steps involved in blood clotting
and wound healing