LINES OF DEFENSE
Physical Barriers, Innate Immunity,
Acquired Immunity
LINE OF DEFENSE
FIRST LINE OF DEFENSE:
PHYSICAL BARRIERS
Skin and Mucosal
Membrane
The skin has a pH of 3–5
● It produces acids that kill
bacteria.
● Oil and sweat gland
secretions make growth of
pathogens on the skin
difficult.
● The outer layer of skin cells
also continually flake off
carrying pathogens with them
Skin and Mucosal
Membrane
Mucous membranes produce
a viscous fluid called mucus
that traps
pathogens and foreign
substances. Also contains
Lysosomes
Defence in the respiratory
tract is aided by cells with
cilia (singular: cilium) that
sweep mucus and trapped
particles
Cilia in Lungs
WHAT HAPPENS WHEN PATHOGENS
GET PAST PHYSICAL BARRIERS?
–>SECOND LINE OF DEFENSE:
INNATE CELLS
(Phagocytosis, Natural Killer cells,
Inflammation, Fever)
PHAGOCYTES
Pathogen is detected by a class of
WBCs called PHAGOCYTES.
●
Recognise pathogens using the
receptors present on the white
blood cell’s surface.
●
The pathogen is engulfed in a
vacuole
●
Vesicles containing lysozyme and
other enzymes fuse with the
vacuole, release their contents and
destroy the pathogen.
Neutrophils are a type of
granulocyte normally
found in the bloodstream.
They are the most
abundant type of
phagocyte and the first
responder to
inflammation
Neutrophils leave the
bloodstream and reach the site
of an infection by squeezing
into the tissues. They rapidly
engulf invaders
Macrophages are tissue
monocytes- they travel in
the bloodstream and are
attracted to infection sites
where they move into
tissues.
They are also stationed in
the spleen and lymph
nodes.
Macrophages act as
scavengers and digest blood
cells that have come to the
end of their life-span, dead
cells and debris
ANTIGEN PRESENTATION
Sometimes macrophages don’t fully digest pathogens.
They can return part of the ingested pathogen to their
cell surface into transmembrane proteins called MHC
Class II
It is displayed to other cells to produce antibodies and
attack the pathogen
–>SECOND LINE OF DEFENSE:
INNATE CHEMICALS
(Antimicrobial peptides and
proteins)
CHEMICAL INNATE IMMUNE RESPONSE
The chemicals are:
1. Interferons
2. Complement system proteins
3. Cytokines.
INTERFERONS
Interferons are glycoproteins that
inhibit the replication of viruses.
Within hours of the onset of a viral
infection, interferons are produced by
the virus- infected cell.
Interferons cause neighbouring
uninfected cells to produce substances
to stop themselves from being
infected and inhibit viral replication.
High concentrations of interferon
limit the spread of the virus until a
further immunological response can
be mounted.
COMPLEMENT
PROTEINS
●
Complement proteins circulate
in the blood plasma in an
inactive form.
They are activated when they
encounter antigens from pathogens
such as bacterial cell walls
Complement proteins join together
to form a protein complex that
punches holes in bacterial cell walls
and plasma membranes
●
These proteins ALSO attract
phagocytes to the infection site
and trigger inflammation
CYTOKINES
Cytokines produced mainly
by T-cells and macrophages
can stimulate the
proliferation, maturation
AND function of the other
cells involved in defence.
→SECOND LINE OF
DEFENSE: INNATE CELLS
AND CHEMICALS
INFLAMMATORY RESPONSE AND
FEVER
MAST CELLS AND HISTAMINE
When a pathogen gets passed the mucosal membrane, mast cells release
histamine.
Mast cells are a type of connective tissue cell that store histamine. Histamine
causes:
1. Blood capillaries dilate and become more permeable or leaky
2. Results in redness and heat in the inflamed area.
3. Complement proteins and antibodies leave the blood and move to tissues
4. Macrophages and neutrophils are activated and they are attracted to the
infection site
5. Capillaries leak fluid into surrounding tissues which causes swelling
LOCALIZE VS SYSTEMIC INFLAMMATION
In a localized inflammation, pus is typically formed. This is a
fluid mixture of living white blood cells, dead microbes and
cell debris that accumulates at the infection site.
A severe injury (e.g. a stab or gunshot wound), tonsillitis, and
appendicitis can cause a severe systemic inflammatory
response. Systemic means a response throughout the entire
body
FEVER
A fever is caused by special substances called pyrogens which are
released by activated macrophages and are a metabolic product of
pathogens
Fevers can cause chills, body aches, a rise in blood pressure and
increased capillary permeability. Bacteria can also produce
pyrogens
Fevers induce stress on pathogens and infected cells directly
→SECOND LINE OF
DEFENSE: INNATE CELLS
NATURAL KILLER CELLS
NATURAL KILLER CELLS
● Natural killer cells are lymphocytes that are able to recognise and
destroy virus-infected cells and cells that have mutated to form
tumours
● All body cells have a protein on their surface called a class I
major histocompatibility complex (MHC) marker molecule
● Virus-infected cells and tumour cells stop expressing class I MHC
molecules on their cell surfaces and the natural killer cells attack
these cells releasing vesicles that contain pore-forming proteins
which result in lysis of bacteria
UNLIKE OTHER
LYMPHOCYTES,
THESE
LYMPHOCYTES
HAVE NO
MEMORY