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The Immune System
Learning objective
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Describe the role of the T cells (T lymphocytes)
in cell-mediated immunity
Describe the role of B cells (B lymphocytes) in
humoral immunity
Immunity
“Free from burden”
The ability of an organism to recognize
and defend itself against specific
pathogens or antigens
What are antigens?
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Every cell has unique proteins on its
surface called antigens.
The antigens on the microorganisms
which get into your body are different
to the ones on your own cells.
Your immune system recognises them as
different and produces antibodies as
part of the body’s
defence.
Human Body
Antigens
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Antigens as part of the cell–surface membrane
of invading cells
pathogen
= antigen
The Immune system is the third line of
defense against infection
Nonspecific defence
mechanisms
First line of
defence
Second line of
defence
Physical
barriers
Phagocytosis
(skin)
Specific defence
mechanisms
(Immune system)
Third line of defence
Cell-mediated
response
Humoral
response
(T Lymphocytes) (B lymphocytes)
Lymphocyte production
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Stem cells in the bone marrow and foetal
liver give rise to T lymphocytes (T cells)
and B lymphocytes (B cells)
B Cells
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B cells mature in the bone in the shaft
of the long bones (e.g. The femur).
The migrate from there to the
lymphatic organs (the lymph nodes and
the spleen).
T Cells
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T cells leave the bone marrow and collect
in the thymus gland.
T Cells
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The thymus gland is located above the
heart on the windpipe.
It is a large in infants, but regresses
with age.
Immature T cells move from the bone
marrow to the thymus to mature before
migrating to other lymphatic organs
Cell-mediated immunity
Involves T cells which defend against:
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Intracellular bacteria and viruses
Protozoa, fungi, flatworms and
roundworms
Cancerous cells and transplanted foreign
tissue
Cell-mediated immunity
T cells are activated when:
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Phagocytes that have engulfed and broken
down a pathogen present some of the
pathogens antigens on their cell-surface
membrane,
Body cells invaded by viruses present viral
antigens on their cell surface membrane,
Cancer cells present antigens on their cell
surface membrane.
Cell-mediated immunity
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These are termed antigen-presenting cells
because they present antigens of other cells
on their cell- surface membrane.
As T cells will only respond to antigens
attached to a body cell, this type of response
is called cell-mediated immunity.
Cell-mediated immunity
Cytotoxic T cells
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T cells kill body cells infected by pathogens.
They produce a protein that makes holes in
the cell-surface membrane, so that the cell
becomes freely permeable to substances and
dies as a result.
Most effective against viruses. As viruses
need living cells to reproduce, this stops them
from multiplying.
Humoral immunity
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Humoral immunity is so named because it is
associated with the serum (non-cellular part
of the blood) , involving substances found in
the ‘humours’, or extracellular body fluids.
Humoral immunity
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It involves B cells secreting antibodies which
recognise and bind to antigens and protect us
against circulating viruses, and bacteria and
their toxins (free antigens).
As many as 10 million different types of B cell
develop in each of us and each one recognises
one specific antigen.
Humoral immunity
Plasma cells
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These secrete antibodies specific to the
antigen that stimulated their development
Each plasma cell lives for only a few days, but
can produce 2000 antibodies per second
Memory cells
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These are retained in the lymph nodes to
provide future immunity.
In the event of a second infection, B memory
cells react more quickly and vigorously than
the initial B cell reaction.
Antigenic variability
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Diseases like chickenpox and measles are of a
single type and easily recognised.
Influenza viruses have over 100 different
strains. This is known as antigenic variability.