Learning Objectives
By the end of this topic, you should be able to:
• explain the different types of specific cellular defences in the human body.
• explain the action of lymphocytes in response to infection or tissue damage.
• explain the immune surveillance theory.
• explain the clonal selection theory.
• explain the role of phagocytes, T lymphocytes and B lymphocytes during antigen
presentation.
• list the different types of lymphocytes.
• explain 'self' and 'non-self' recognition in the immune system
• list the different types of T cells;
• explain the functions of cytotoxic T cells;
• explain the functions of T helper cells.
• state the functions of B lymphocytes.
• describe what is meant by immunological memory;
• explain that T and B lymphocytes can form memory cells;
• state the benefits of having memory cells;
• understand what primary and secondary response to infection is.
1. Specific Cellular Defences
If the non-specific defences fail to get rid of the invading pathogen our
body switches to its’ third line of defence: the specific immune response!
This is brought about by lymphocytes of which there are 2 main types:
- T lymphocytes (T cells)
- B lymphocytes (B cells)
Immune Surveillance
Immune surveillance is a theory to explain how the
immune system continuously patrols the body not only to
recognise and destroy invading pathogens but also to
destroy host cells that become cancerous via the detection
of new antigens (a molecular marker found on the surface
of cells) by T lymphocytes.
A group of white blood cells called leucocytes are the
main cells involved in this process.
They release cytokines into the blood stream to recruit
large numbers of phagocytes and T cells to the site of
injury to fight off the invading microorganism.
Clonal Selection
The clonal selection theory states that the body has large number of
lymphocytes each with a single type of membrane receptor specific for
one antigen. Essentially each lymphocyte can only be activated by one
type of antigen and when this occurs the lymphocyte will respond by
dividing repeatedly to form a ‘clonal population’ of identical
lymphocytes.
This is the process of clonal selection.
2. Antigen Presentation
T Lymphocytes
There are two main groups of T cells in the body which are involved in
the specific response and antigen presentation;- helper T cells (TH cells)
- cytotoxic T cells ( TC cells)
Helper T cells can be regarded as the main regulators of the immune
system. Their main task is to activate B cells and cytotoxic T cells as
they themselves cannot destroy an infected cell.
It is important that the helper T cells are themselves being activated.
This happens when a phagocyte has engulfed a pathogen and
displays fragments of its antigens on its surface for recognition by the
TH cells- which in turn will trigger the formation of a clone of activated
TH cells and a clone of memory TH cells which will release cytokines to
stimulate cytotoxic T cells and B cells.
This is the process of antigen presentation and is shown in the
diagram below:
Cytotoxic T cells also undergo this clonal activation process once they
recognises a specific antigen and will move to the site of infection to
attack infected cells by releasing chemicals to cause lysis or induce
apoptosis.
B Lymphocytes
The presence of a foreign antigen stimulates the production of
antibodies by B cells. Antibodies are Y-shaped protein molecules with
receptor binding sites which are specific to a particular antigen as
shown below:
An antibody response can occur as a direct response upon contact
between a B cell and the antigen, however in most cases it is a TH cell
that is involved by releasing cytokines to stimulate a clone of
activated B cells for immediate antibody production and a clone of
memory B cells for future use.
Antibodies work by inactivating the toxic component of the foreign
pathogen upon the establishment of an antibody-antigen complex in
order for phagocytosis to occur.
3. Self vs Non-Self Antigens
A unique property of the body's specific cellular defence is its ability to
differentiate between the body's own cells, recognised as “self,” and
foreign cells, or “non-self.” Moreover, every person, with the exception of
identical twins, has a unique antigen signature recognised only by their
own cells as being 'self' and therefore safe.
THINK
What might be the expected immune response in a patient
who has received a blood transfusion or an organ transplant?
Can we control our natural response? Note your ideas:
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Cells bearing non-self antigens are attacked by the immune system.
When this system breaks down and the immune system stops identifying
'self' antigens as safe, autoimmune disorders occur.
Choose one of the autoimmune diseases listed below and
produce either a written report (300-400 word) or a
PowerPoint presentation on the disease you have chosen.
- Rheumatoid Arthritis
- Type 1 Diabetes
- Multiple Sclerosis
THINK
Based on your knowledge so far of the immune system, what
do you think happens during an allergic reaction?
Discuss your ideas with your partner and note them in the
space provided below.
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4. Immunological Memory
Primary Response:
In the primary response several days elapse before a significant
number of antibodies appear in the bloodstream.
It is often not quick or large enough to prevent the person suffering the
symptoms of the disease.
The primary immune response can take between 10-17 days from
initial exposure until selected lymphocytes generate the maximum
effector cell response. During this time the affected individual may
become ill.
Secondary Response:
If the person survives the initial infection and is exposed to the same
antigen at a later time, the secondary response is initiated.
This time antibody production:
• is much more rapid;
• is at a higher concentration;
• is maintained for a longer time;
• has a greater affinity for the antigen.
Memory cells can proliferate and differentiate rapidly if they meet the
same antigen. As a result the person usually does not suffer the disease
symptoms.
Below is a summary of the primary and secondary responses:
Summary
• Immune surveillance explains:
– how some white blood cell constantly circulate and monitoring the tissues,
– when these tissues become damaged or invaded, a variety of cells release
cytokines,
– the process recruit specific white blood cells to the site of infection or tissue
damage.
• An antigen is defined as a molecule recognised as foreign by an organism and
which elicits an immune response.
• The clonal selection theory explains that:
– the body has a wide range of lymphocytes, each consists of a single type of
membrane receptor specific for one antigen;
– if a receptor is activated by the binding of an antigen, the lymphocyte
repeatedly divides, resulting in a clonal population of lymphocytes.
• The role of phagocytes, T lymphocytes and B lymphocytes during antigen
presentation is:
– phagocytes engulf pathogens and display part of the antigen on their surface,
– helper T cells, the major regulators of the immune system, are activated by
the phagocyte antigen,
– active B cells, cytotoxic T cells as well as other immune cells are activated by
the helper T cell.
• Cells bearing non-self antigens are attacked by the immune system.
• 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.
• Failure in regulation of the immune system leads to a T lymphocyte immune
response to self cells (autoimmune disease).
• There are four possible blood groups among humans: A, B, AB and O.
• The antigens on the red blood cells of different blood groups are as follows:
- blood group individuals A possess A antigens;
– blood group B individuals possess B antigens;
– blood group AB individuals possess both A and B antigens;
– blood group O individuals possess no antigens - since they have no antigens
to stimulate an immune response blood group O can be given to any other
blood group and is known as the universal donor.
• If exposed to different blood groups the following antibodies will be produced:
– A individuals will produce anti-B antibodies;
– B individuals will produce anti-A antibodies;
– AB individuals will not produce any antibodies - they are said to be universal
recipients because they can receive blood from any other blood group without
producing antibodies;
• Individuals will produce both types of antibody and can therefore only receive blood
from other O individuals.
• Autoimmunity occurs when the immune system fails to recognise its own cells
as 'self' and treats them as if they were foreign, 'non-self' cells by attacking and
destroying them.
• An allergy occurs when the immune system makes a mistake and misidentifies a
harmless substance as a harmful one.
– the B cells cause the body to produce large quantities of antibodies;
– these attach themselves to specialised cells in the connective tissue;
– this sensitises the body to the allergen and causes an allergic reaction when
the allergen next enters the body.
• T lymphocytes are produced in the bone marrow and mature within the thymus
gland.
• Cytotoxic T cells detect and kill cells in the body which are harbouring pathogens.
• They do this by secreting toxic chemicals into the cells.
• Because the T cells themselves are directly involved in the immune response, this
is called the cellular response.
• T helper cells produce chemicals which activate B lymphocytes so that they
produce antibodies.
• They also increase the rate of phagocytosis.
• Antibodies and antigens bind together. When this happens, a complex forms
making the antigen harmless.
• B lymphocytes are produced and mature in the bone marrow and carried by the
blood to the lymph nodes.
• When they come into contact with a foreign antigen the B cells differentiate and
secrete large quantities of antibodies into the lymph and blood.
• Because the antibodies act at a distance from the parent B-lymphocytes, this
response is known as the humoral response.
• Memory cells are formed if some of the T and B lymphocytes produced in response
to antigens by clonal selection can survive long term.
• Two clones of cells are formed:
– effector cells which are short-lived and responsible for the primary immune
response;
– memory cells are long-lived and responsible for the secondary immune
response.
• The primary immune response occurs when the body is infected by a pathogen for
the first time.
• The secondary immune response occurs when the body is subsequently infected
by the same pathogen.
• The primary response is slower and produces fewer antibodies than the secondary
response.