The Immune System
Specific Immunity
What You Should Know
• Immune surveillance
• A range of white blood cells constantly circulate
monitoring the tissues.
• If tissues become damaged or invaded, cells release
cytokines which increase blood flow resulting in specific
white blood cells accumulating at the site of infection or
tissue damage.
• Clonal selection theory.
• Lymphocytes have a single type of membrane receptor
specific for one antigen.
• Antigen binding leads to repeated lymphocyte division
resulting in a clonal population of lymphocytes.
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Immune Surveillance
• A range of different types of
white blood cells continuously
circulate in the blood
monitoring the state of the
tissues
• If damage is detected some of
these white blood cells release
cytokines into the blood
• This attracts large numbers of
phagocytes and T cells to the
damaged or infected area
• Often white blood cells
squeeze through the pores in
the capillaries to gain access to
the tissue
Lymphocytes on patrol
White blood cells, called
lymphocytes, move through the
bloodstream alongside red blood
cells.
Clonal Selection Theory
• Any foreign molecule that is able
to elicit a specific response from
the immune system is referred to
as an antigen
• Viruses, bacteria, toxins etc.
• The body has an enormous
number of different
lymphocytes
(continued…)
• Each lymphocyte has a
single type of antigen
receptor on the surface
of its membrane
• The antigen receptor is
specific for one antigen
• Each lymphocyte is able
to attach to and be
activated by one type of
antigen
(continued…!)
• The lymphocyte then
divides repeatedly to
form a clonal population
of identical lymphocytes
• This is called clonal
selection
Recognition of Self and Non-Self
What You Should Know
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Lymphocytes respond specifically to antigens on foreign cells, cells infected by pathogens and
toxins released by pathogens.
T-lymphocytes have specific surface proteins that allow them to distinguish between the
surface molecules of the body’s own cells and cells with foreign molecules on their surface.
Immune system regulation failure leads to T-lymphocyte immune response to self antigens
(auto immune disease).
Allergy is a hypersensitive B- lymphocyte response to an antigen that is normally harmless
T-lymphocytes
One group of T-lymphocytes destroy infected cells by inducing apoptosis.
Another group of T-lymphocytes secrete cytokines that activate B lymphocytes and
phagocytes.
When pathogens infect tissue, some phagocytes capture the pathogen and display fragments
of its antigens on their surface.
These antigen presenting cells activate the production of a clone of T-lymphocytes that move
to the site of infection under the direction of cytokines.
B-lymphocytes
Each B-lymphocyte clone produces a specific antibody molecule that will recognise a specific
antigen surface molecule on a pathogen or a toxin.
Antigen-antibody complexes may inactivate a pathogen or toxin or render it more susceptible
to phagocytosis.
In other cases the antigen-antibody complex stimulates a response which results in cell lysis.
B-lymphocytes activated by antigen presenting cells and T-lymphocytes produce a clone of Blymphocytes that secrete antibodies into the lymph and blood where they make their way to
the infected area.
Lymphocytes
• Every individual has a unique
‘antigen signature’
• It is vital that a person’s
lymphocytes do not regard the
person’s cell surface proteins
as antigens and attack them
• Lymphocytes respond
specifically to
– antigens on foreign cells
– cells infected by pathogens
– toxins released by
pathogens
T Lymphocytes
• T-lymphocytes have specific
surface proteins that allow
them to distinguish between
the surface molecules of the
body’s own cells and cells
with foreign molecules on
their surface
• ‘self’ molecules on the
surface- no action
• ‘non-self’ molecules- immune
response initiated
Those lymphocytes that bind to
antigens from the body's own
tissues are destroyed
Auto Immune Disease
• The failure to recognise self-antigens can result in
autoimmune disease
• The T lymphocytes attack the body’s own cells
• Examples include
– Type 1 diabetes
– Rheumatoid arthritis
– Multiple sclerosis
– Celiac disease
(see textbook for more detail)
Allergy
• An allergy is a hypersensitivity disorder of the immune
system
• Sometimes the immune system over reacts to harmless
substances e.g. pollen, dust.
• When the immune system over reacts B lymphocytes
are activated producing antibodies which attach to the
mast cells in the connective tissue causing the release
of histamine
• Excessive histamine stimulates the inflammatory
response
T Lymphocytes
(2 types)
• 1. Cytotoxic T cells (TC) destroy infected cells by
inducing apoptosis
• 2. Helper T (TH) cells secrete cytokines that
activate phagocytes, cytotoxic T cells and B
lymphocytes
Antigen Presenting Cell
• After a phagocyte destroys an
invading pathogen it presents
fragments of its antigens on its cell
surface
• The phagocyte is now referred to as
an antigen presenting cell
• A specific helper T cell then binds
to the antigen on the antigen
presenting cell
• The helper T cell becomes activated
and produces a clone of activated
helper cells
• The helper T cells secrete cytokines
which activate B cells to secrete
antibodies and macrophages to
destroy ingested microbes. They
also activate cytotoxic T cells to kill
infected target cells.
B Lymphocytes
• B-lymphocytes activated by
antigen presenting cells and Tlymphocytes produce a clone
of B-lymphocytes that secrete
antibodies into the lymph and
blood where they make their
way to the infected area
• Each B-lymphocyte clone
produces a specific antibody
molecule that will recognise a
specific antigen surface
molecule on a pathogen or a
toxin.
The Action of B Lymphocyte
• Antigen-antibody
complexes may inactivate
a pathogen or toxin or
render it more
susceptible to
phagocytosis
The Action of B Lymphocyte
• In other cases
the antigenantibody complex
stimulates the
activation of
proteins which
result in cell lysis
Immunological Memory
What You Should Know
• Immunological memory.
• Some T- and B-lymphocytes produced in
response to antigens by clonal selection
survive long-term as memory cells.
• A secondary exposure to the same
antigen rapidly gives rise to a new clone of
lymphocytes producing a rapid and greater
immunological response.
Immunological Memory
• Towards the end of each battle to
stop an infection, some T-cells and Bcells turn into Memory T-cells and
Memory B-cells
• These cells remember the virus or
bacteria they just fought
• These cells live in the body for a long
time, even after all the viruses from
the first infection have been
destroyed
• They stay in the ready-mode to
quickly recognize and attack any
returning virus or bacteria
Secondary Response
• The first time your body fights
a virus, it can take up to 15 days
to make enough antibodies to
get rid of it
• With the help of Memory Bcells, the second time your body
sees that virus, it can do the
same in thing 5 days
• It also makes 100 times more
antibodies than it did the first
time
• The faster your body makes
antibodies, the quicker the
virus can be destroyed
• With the help of Memory Bcells, you might get rid of it
before you even feel sick
• This is called gaining immunity