Download Specific Cellular Defences - Smithycroft Secondary School

Unit 4 - Immunology and Public Health
CfE Higher Human Biology
22. Specific Cellular Defences
Learning Intentions
I can state that 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.
I can state that each B-lymphocyte clone produces a specific antibody molecule that
will recognise a specific antigen surface molecule on a pathogen or a toxin. Antigenantibody 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 B-lymphocytes that secrete antibodies into the
lymph and blood where they make their way to the infected area.
I can describe immunological memory.
I can state that 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
specific cellular defences-lymphocytes(the third
line of defence).
• The third line of defence is brought about by
lymphocytes derived from stem cells in bone marrow.
• The thymus gland is a gland in the chest cavity and
some lymphocytes pass to this gland to develop into T
lymphocytes(T cells).
• Those that stay and mature in bone marrow become B
lymphocytes (B cells).
T-lymphocytes
There are two different types of Tlymphocytes or T-cells.
- helper T cells (TH cells)
- cytotoxic T cells (TC cells)
Helper T cells become activated when they have
come into contact with a foreign antigen.
They secrete cytokines activate phagocytosis,
Tc cells and B cells.
Cytotoxic T cells cause apoptosis of an infected
cell.
TH Cells
Antigen presenting cell
.
TH Cells
As seen before, there is a
vast pool of different TH
cells with different forms
of antigen receptors. One
of them will be specific to
the antigen.
When the phagocyte has
engulfed the foreign cell
and has presented the
foreign antigen, one of
the TH cells will be able to
bind with it.
TH Cells
Once one version of the TH cell is activated, it goes
on to multiply to give either;
- further clones of activated TH cells
- clones of memory TH cells
The activated TH cells release cytokines which will
stimulate cytotoxic T cells (Tc cells) and also
stimulate B cells
To see animation about Th click here
Antigen presenting cell.
• When a phagocyte has captured and
destroyed an invading pathogen it
usually presents fragments of the
pathogen’s antigens on its surface.
• This kind of phagocyte is called an
antigen presenting cell.
• Other cells in the body also become
antigen presenting cells.
Antigen presenting cell.
TC cells
Cytotoxic T cells are also activated in a similar
way, where the specific antigen receptor
binds to the antigen on the presenting cell.
They go on to multiply to give either;
- clones of the activated TC cells
- clones of memory TC cells
The activated TC cells go on, under the action
of cytokines from TH cells, to destroy
infected cells
Action of cytotoxic cells.
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In the body’s pool of cytotoxic T cells there is a type of Tc cell that has copies of
one type of antigen receptor on its surface that are specific to and are able to bind
to the type of foreign antigen on the surface of an antigen- presenting
phagocyte.
This binding process results in the Tc cell becoming activated by the antigen
presenting cell and then proliferating and differentiating.
A clone of activated Tc cells and a clone of memory Tc cells are made.
The activated Tc cells go to the site of the infection under the influence of
cytokines released by helper T cells and attack infected cells.
Some types of Tc cells kill the infected antigen presenting cells by inducing them to
undergo apoptosis.
Once a Tc cell has killed a target cell it leaves the cell and moves on to another
infected cell.
An infected cell shrinks but its membrane is not destroyed by apoptosis but its cell
contents stay enclosed and are not dispersed.
A phagocyte engulfs and digests the infected cell.
Other Tc cells recognise antigens on the surface of cancer cells and attack them.
Action of cytotoxic cells.
Destruction of infected cells
Apoptosis is where cells are programmed to self
destruct.
The TC cells bind to an infected cell as they have copies
of the foreign antigen on their surface.
The TC cells release chemicals which perforate the cell
membrane allowing further chemicals access to the
cell causing the DNA and proteins to be broken down.
The cell now slowly shrinks and dies, allowing a
phagocyte to engulf and digest it.
This means the cell remains whole so the foreign
pathogen is not released to infect more cells.
TC cells and cancer
TC cells are also able to recognise antigens
found on the surface of cancer cells.
They are then able to bring about the lysis of
large cancer cells.
B-lymphocytes
B lymphocytes are able to recognise foreign
antigens and engulf them.
They then
display the
antigens on
their surface
waiting for a
TH cell to
release
cytokines and
activate it.
Some will become clone antibody-producing B
cells, others become cloned memory B cells
Activated B lymphocytes
Activated lymphocytes can produce a protein specific to the
antigen called antibodies.
The antibodies are able to bind to the antigen creating an
antigen-antibody complex which makes it more
susceptible to phagocytosis
An antibody is
a y shaped
protein
molecule
where each
arm has a
receptor
binding site
specific to a
particular
antigen.
Antibody action
The binding of the antibodies causes the inactivation
of the pathogen (or the toxin it produces) and
pathogen to become more susceptible to
phagocytosis. In some cases the formation of the
complex itself stimulates the activation of proteins
that bring about lysis of the pathogen.
Production of antibodies-like a protein factory.
1. Each clone of B cells produce one type of
antibody.
2. These antibodies will be specific to one type of
surface antigen molecule on the surface of a
pathogenic cell or toxin.
3. Each B cell produces about 2000 antibody
molecules per second during its 4-5 day life span.
4. Once released into the blood and lymph system
the antibodies are transported around the body
and head to the infected area.
Immunological memory
When a pathogen infects the body, it takes
time to raise an immune response and select
the correct T and B cells (clonal selection)
and produce antibodies.
This is called the primary response, and very
often it is too slow to prevent the pathogen
causing illness.
However, if the person survives, and they come
across the pathogen a second time they have
memory TC , TH and B cells.
Memory Cells
When the foreign antigen is recognised by these
memory cells, they very quickly proliferate and
form many clones of both T cells and antibodyproducing B cells.
This means that their are more antibodies
produced faster and for a longer duration.
This is the secondary response.
Primary and secondary response
and immunological memory.
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On first being exposed to a pathogen it takes a certain length of time
for antibodies to it to appear in the blood stream.
This is called the primary response.
Sometimes there are not enough antibodies made to prevent the person
from suffering the disease.
If the person survives and meets this same pathogen again then the
production of antibodies will be much more rapid and of a higher
concentration that is maintained for a longer time.
This is called the secondary response.
As a result the disease is usually prevented.
The secondary response is possible due to the presence of memory
cells.
These are B and T lymphocytes specific to the antigen and produced in
response to it by clonal selection following the body’s first exposure to
it.
When exposed a second time the memory cells proliferate and
differentiate producing clones of T cells and B cells and antibody
forming B cells.
Antigen engulfed and
presented on its surface
TC cell activated
Formation
of clone
memory
TC cell
Formation
of cloned
activated
TC cells
Infected cell
destroyed
TH cell activated
Formation
of clone
memory
TH cell
Formation
of cloned
activated
TH cell
Produce cytokines
to stimulate
B cell activated
Formation
of clone
memory B
cells
Formation
of cloned
antibody
producing
B cells
Antibodies
produced
Questions
7) How does a TC cell lead to the destruction of an
infected cell?
8) a) What does an activated B cell produce?
b) How do these molecules bring about destruction of a
pathogen?
9) What term is used to describe the first infection of
the body by a pathogen?
10) If the body is re-infected at a later date, what is
this known as?
11) Which cells are central to being able to fight off reinfection?
12) How does the immune response to the second
infection compare to the first?
Chapter 22 specific cellular
defences.
1.
Give another name for the third line of defence against pathogens in
the body.
2. Name the two types of lymphocytes involved in this line of defence
and say where each type originates from.
3. From which cells did all of these lymphocytes originally develop from?
4. What is meant by ‘immune surveillance’?
5. What is an ‘antigen’?
6. Name 5 things that can all act as antigens.
7. What is an ‘antigen receptor’? Say what is special about it using the
words ‘selected’, ‘clonal population’, and ‘clonal selection’.
8. Describe why the process of ‘weeding out’ any lymphocyte with an
antigen receptor that would fit a body cell surface protein is very
important.
9. What is autoimmunity and name 2 diseases of this type.
10. When would the body exhibit an allergic reaction?
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Chapter 22 specific cellular defences continued
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1.
Name the two groups of T lymphocytes produced by the human body
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4.
Describe in general how each type of T cell operates-top paragraph page 321.
What is an antigen presenting cell? Figure 22.13 page 321 will help you describe it.
Look at figure 22.14 page 321 and describe numbered stages of the activation of a
Th cell.
Name the cells that stimulate other cells including Tc cells and B cells.
Look at figure 22.15 page 322 and describe numbered stages of the activation of a
Tc cell.
Look at figure 22.16 page 322 and describe numbered stages of the destruction of
infected cells by Tc cells.
Describe how other Tc cells recognise and destroy cancer cells.
The presence of what triggers the production of antibodies by B lymphocytes?
Describe an antibody and then draw and label one.
Describe the indirect way in which antibodies are produced by B cells and with the
help of Th cell by listing the numbered stages from figure 22.19 page 323 .
Describe the action of antibodies by listing numbered stages from figure22.20
page 324(production of antibodies).
Write about immunological memory.
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