IMMUNE SYSTEM
THE INNATE IMMUNE SYSTEM
Each of us has many inborn defences
against infection. These defences
include external barriers such as the
skin and eye surfaces (below), the
inflammatory response (right), and
the action of white blood cells called
phagocytes (below right). Others
include a substance called
complement (which is activated by and
attacks bacteria) and another called
i n t e r f e r o n
(which has antiviral effects).
All of these defences are nonspecific
and quick-acting. By contrast, the
adaptive immune system (see overleaf)
mounts specific attacks against
particular microbes. These cells are
most effective on second exposure
to the organisms.
The two parts of the immune system
worktogether; antibodies produced by
the adaptive immune system assist
phagocyte action.
Physical and chemical barriers
These barriers, briefly described below,
provide the first line of defence against
harmful microbes (bacteria, viruses,
and fungi).
E y e s
T e a r s p r o d u c e d b y t h e
la c r i m a l a p p a r a t u s h e l p to
w a s h a w a y m i c r o o r g a n i s m s ;
t e a r s c o n t a i n a n e n z y m e
c a l l e d l y s o z y m e , w h ic h c a n
d e s t r o y b a c t e r i a .
M o u t h
THE INFLAMMATORY RESPONSE
Ifmicrobes breakthrough the body’s outermost barriers, inflammation is the
second line ofdefence. Chemicals such as histamine are released, prompting
the effects shown below, including the attraction ofphagocytes to the
microorganisms and the release ofsubstances such as complement.
The symptoms ofinflammation are redness, pain, swelling, and heat.
Inflammatory process
Following tissue injury (here due to a splinter
in the skin) and the entry of bacteria or other
microbes, blood vessels in the area widen and
there is an increased leakage offluid from the
blood into the tissues. These reactions allow
easier access for immune system components
that will fight the invaders, including
phagocytes and soluble factors (such as the
group of substances known as complement).
Breast-feeding
Antibodies (proteins with a protective
role) formed bythe mother against particular
microbes are transferred to the baby in breast
milk. Breast-feeding provides some extra
immunity until the baby can form his or her
own specific antibodies.
N o s e
H a ir s in t h e n o s e h e l p t o
p r e v e n t m i c r o o r g a n i s m s
f r o m e n t e r i n g o n d u s t
p a r t i c l e s . T h e p r o c e s s
o f e x p e l l i n g d u s t a n d
m i c r o b e s i s a s s i s t e d b y
t h e s n e e z e r e fl e x .
R e s p i r a t o r y t r a c t
M u c u s s e c r e t e d b y c e l l s
lin in g t h e t h r o a t , w i n d p i p e ,
a n d b r o n c h i t r a p s m i c r o b e s ,
w h i c h a r e t h e n s w e p t a w a y
b y c ilia ( h a i r s o n c e l l s in
t h e lin in g ) o r e n g u l f e d b y
p h a g o c y t e s ( t y p e s o f w h it e
c e l l s ) . T h e c o u g h r e f l e x a l s o
h e l p s t o e x p e l m i c r o b e s .
S t o m a c h a n d i n t e s t i n e s
S t o m a c h a c i d d e s t r o y s t h e
v a s t m a j o r i t y o f m i c r o b e s .
T h e i n t e s t i n e s c o n t a in
h a r m l e s s t y p e s o f b a c t e r i a
( c o m m e n s a l s ) t h a t c o m p e t e
w it h a n d c o n t r o l t h e
h a r m f u l m i c r o o r g a n i s m s .
G e n i t o - u r i n a r y
sys
t e m
T h e v a g in a a n d u r e t h r a a l s o
c o n t a i n c o m m e n s a l s a n d
a r e p r o t e c t e d b y m u c u s .
S k in
I n t a c t s k i n p r o v i d e s a n
e f f e c t i v e b a r r i e r a g a i n s t
m o s t m i c r o b e s . T h e
s e b a c e o u s g l a n d s s e c r e t e
c h e m i c a l s t h a t a r e h i g h l y
t o x ic t o m a n y b a c t e r i a .
ACTION OF PHAGOCYTES
These white blood cells are attracted
to sites of infection, where they adhere
to, engulf, and digest microorganisms
and debris.
P h a g o c y t e
N u c le u s
B a c t e r iu m
1
Adherence
The phagocyte comes into contact
with a microbe and recognizes it as foreign.
This process is assisted by chemicals released
during inflammation.
L y s o s o m e s
M e m b r a n e
P o u c h
f o r m i n g
2
Ingestion
The phagocyte engulfs the microbe
in a pouch formed in its membrane. Fluid-filled
particles, lysosomes, move towards the microbe.
E n z y m e s
r e l e a s e d in t o
p o u c h
B a c t e r iu m
3
Digestion
Enzymeswithin the lysosomesare
released into the pouch to help digest the
microbe. Debris from this process is later ejected.
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