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The Defence System
Chapter 38
Pathogens
 Pathogens are disease causing
organisms
 The human body has 2 ways of
defending against pathogens
1. The GENERAL defence system
2. The SPECIFIC defence system
The General Defence System
Acts as a barrier to pathogens entering the body
Barrier
Reason
Skin
Secretes enzymes from the
sebaceous glands that kill
bacteria. It is a physical barrier
Lines breathing, reproductive &
digestive tracts, Its sticky & traps
pathogens before they enter the
body
Ingest pathogens
Mucous
membrane
Phagocytic WBC
Specific Defence System
This system is activated when pathogens
get past the general defence system
Organs of the immune system that store
WBC’s called lymphocytes and monocytes
include the lymphatic vessels, tonsils,
spleen lymph nodes
Antigens
An antigen is a foreign molecule that
causes antibody production
Antigens are located on:
bacterial cell walls
viral coats
cancerous cells
Antibodies
An antibody is a protein produced by white
blood cells in response to an antigen
Each type of antibody is highly specific to
a single antigen
Antibodies inactivate antigens and allow
them to be destroyed
Monocytes & Lymphocytes
MONOCYTES:
Are WBC formed in the bone marrow
They engulf foreign bodies
They move into the lymphatic system eg
lymph nodes, vessels ,spleen & thymus
gland
LYMPHOCYTES:
WBC formed in the bone marrow
Move into lymphatic system--- produce
antibodies
2 types – B cells & T cells
Advanced Study of Lymphocytes
Higher Level Only
B – lymphocytes (B-cells)
Become active in the bone marrow
Each B cell produces only one type of
antibody
When a B cell comes in contact with an
antigen it divides into thousands of
identical B cells ( called plasma cells)
These plasma cells produce antibodies
that combine with and deactivate the
antigen
THE IMMUNE
RESPONSE
Plasma cells only live for a few days and
most die off once infection is over
Some live on forever , these are called
memory B –cells, if the same antigen
enters the body again these B- cells
produce more antibodies much faster than
before, which prevents us being infected
with the same antigen more than once
T- lymphocytes ( T-cells)



1.
2.
3.
4.
Become active in the thymus gland
Do not produce antibodies
Act in one of 4 ways
Helper T-cells
Killer T- cells
Suppressor T- cells
Memory T-cells
Helper T- cells
Recognise antigens from other white blood
cells
Enlarge and form clones
These stimulate the production of B -cells
Killer T-cells
Destroy abnormal ( infected ) body cells
Recognise antigens on the surface of
infected cells
They release proteins ( perforin) which
form pores in the membrane of target cells
Water & ions flow in through these pores
Infected cells swell & burst
Suppressor T-cells
Control ( suppress) the immune response
Memory Cells
Memory T- cells survive for life and
memorise the immune response
Induced Immunity
higher & ordinary level
 Is the ability to resist disease caused by
infection
 There are 2 types of induced immunity
1. Active immunity
2. Passive immunity
Active Immunity
Means the person produces his/her own
antibodies in response to antigens
It is long term immunity
It develops after a vaccination or an
infection
(artificial)
(natural)
Getting a vaccine
Pathogens are
introduced into
the body
Pathogens
entering body
in a natural
manner eg
getting a cold
Passive Immunity
Occurs when people are given antibodies
to fight disease
They are not made by the person’s own
immune system
It is short term immunity---eg. child getting
antibodies naturally from its mother’s
placenta or breastmilk (natural) OR getting
and injection of foreign antibodies eg
tetanus injection(artificial)
Vaccination
Is an injection of a killed pathogen in order
to stimulate the immune system against
the pathogen, thereby preventing the
disease being suffered Eg. MMR, BCG
IMMUNISATION
Is a process that increases an organisms
reaction to an antigen & therefore
improves its ability to resist or overcome
infection