Name: ___________________________________
Detecting and Responding:
VCE Biology- Unit 3
The Immune Response
1. The human body has a number of levels of defence against disease. What is the first line of defence?
The external barriers of the body are the first line of defence. The skin is relatively tough and impermeable
unless broken. The skin produces its own antiseptic- sebum. An antiseptic in tears protects the eyes. Hairs
and mucus protect the entrance to the respiratory system. Stomach acid has a role in killing pathogens that
enter in food. Urine, itself acidic, helps flush out any pathogens that enter the urethra.
2. How do organisms determine which cells are theirs and which cells are foreign?
Cells in an organism have their own particular molecules on their surfaces. These molecules are usually
made of proteins, carbohydrates, glycoproteins or glycolipids. Cells with different molecules on their
surfaces are recognised as foreign or non-self and are attacked by the immune system.
3. What is the second line of defence? What is meant by the inflammatory response?
The second line of defence is inside the body. It will attempt to neutralise any pathogens that have got past
the first line of defence. Involved are scavenger cells called phagocytes which respond to chemicals
produced by damaged cells. The lymphatic system’s lymph nodes filter the lymph. Remember that lymph is
the excess fluid drained from around cells. Other scavenger cells in the lymph nodes remove any cell
fragments and foreign material.
4. Explain whether the features described above provide specific defence.
The second line of defence is non-specific in that any pathogen or foreign material can be removed. The
scavenger cells involved will attempt to attack any potential threat.
5. What is the role of the specific defence system?
The role of the specific defence system is to combat any threats that have overcome the first two lines of
defence.
6. Distinguish between T cells and B cells. Where is each sort of cell formed and matured?
T cells (T lymphocytes) and B cells (B lymphocytes) are both formed from stem cells in the bone marrow of
long bones. The B cells mature in the bone marrow to be released into the blood where they become
antibody producers. B cells are involved in humoral immunity. The T cells travel to the thymus gland to
complete their development. (T stands for ‘thymus dependent cell’.) The T cells are involved in cellmediated immunity. Some become killer cells.
7. What is the difference between antibodies and antigens? How do they relate to each other?
In terms of the immune system, an antigen is a molecule that provokes the production of antibodies.
Antibodies are the bullets produced by B cells designed to damage pathogens. The molecules on the surface
of pathogens, or for that matter on the body’s own cells, can act as antigens. A specific antigen will provoke
the production of one type of antibody.
8. What are antibodies made of and by what cells are they made?
Antibodies are protein molecule made to precise shapes. A form of the B cell called a plasma cell makes
them. The plasma cells are antibody factories. One type of plasma cell can only produce one specific type of
antibody.
9. Why is the structure of antibodies so important?
Antibody shape is important because it determines the antigen to which it can stick. In a similar way to
enzymes and substrate molecules, there is a lock and key relationship. One antibody type combines with one
antigen type. Thus the defence is specific.
Name: ___________________________________
10. List the major steps involved in humoral immunity.
Humoral immunity:
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VCE Biology- Unit 3
Macrophages engulf pathogens and then display some of the pathogen’s antigens on their surface
(antigen-MHC complex)
T-helper cells are attracted to the macrophages and are activated by the antigen-MHC complex.
T-helper cells stimulate B cell activity
If the appropriate B cell is present with the antibody specific to the microbe antigen it will bind to the
antigen-MHC complex on the macrophage
The B cell divides to form a clone of plasma cells that produce a specific antibody
Also produced are ‘memory cells’ that are identical to the original B cell.
11. List the major steps involved in cell-mediated immunity.
Cell-mediated immune response
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Infected cells display antigen-MHC complex
T-helper cells are activated by the antigen-MHC complex
T-helper cells stimulate cytotoxic T cells to destroy infected cells
12. Explain why most people get mumps or the measles once only. (Assume they were not vaccinated!)
After combating a disease successfully there will be a large number of memory cells. After having mumps
once, the memory cells that ‘remember’ how to make mumps antibody are common. If the mumps virus
invades again, the defence system springs into action more quickly because of the large population of the
specific memory cell. The body can overcome the second attack without symptoms.
13. What is the principle behind active immunity?
With active immunity the body is producing its own antibodies. Immunisation with a vaccine that contains a
harmless form of a microbe can produce immunity without suffering the disease.
14. How does active immunity differ from passive immunity? Which is best?
With passive immunity the body does not produce the needed antibody and there is no immune memory. In
the case of a snakebite where the toxin is fast acting, a dose of the appropriate antivenene (which contains
antibodies specific to the snake toxin) is necessary for the patient’s survival. The body has not got time to
produce the specific antibody required. In cases of active immunity, the body’s own immune system
produces the antibody and memory cells, which in the long run is more effective than passive immunity. In
crisis situations, such as a snakebite, passive immunity is more effective!
15. Describe the different ways that T cells are involved in specific defence.
Like B cells there are many kinds of T cells. Helper T cells recognize foreign antigens and assist B cells.
Some T cells become the killer cells involved in graft rejection after transplant operations. Each person’s
antigens are different and transplant tissue is seen as foreign. The immune system tries to destroy it!
Cytotoxic T cells are involved in destroying cells infected with virus and cancer cells.
16. What is Rhesus incompatibility? How is it overcome?
A particular antigen, the Rhesus antigen (Rh), is present on the red blood cells of most people who are
described as being Rh +ve. Some people (Rh –ve) do not have the antigen and produce the antibody if cells
with the antigen enter the body. If an Rh –ve mother produces a Rh +ve child she may start producing Rh
antibodies. At birth a few of the baby’s Rh +ve cells may get into the mother’s circulatory system. This is a
problem for a second Rh +ve child whose body will be attacked by these antibodies that can cross the
placenta. If the mother is given a dose of Rh antibody immediately after birth of her first child, any of the
baby’s cells can be removed before they provoke the mother’s immune system to produce Rh antibody.
Name: ___________________________________
VCE Biology- Unit 3
17. How is rejection of transplanted organs and autoimmune disease similar?
In autoimmune diseases the body’s immune system is attacking itself. A transplanted organ may be useful
but as far as the immune system is concerned, the tissue is foreign.
18. What is the role of immunosuppressant drugs? Do they have any disadvantages?
Rejection can be reduced by using immunosuppressant drugs which suppress the immune system. The
disadvantage is that the patient will be more susceptible to infectious disease.
19. What is the role of histamines in the body and why do some people take antihistamines?
Histamines are chemicals that are produced by damaged cells. They trigger the non-specific inflammatory
response. In people with a condition like hay fever the cells detect foreign material (e.g. pollen) and respond
over-vigorously. Antihistamines are designed to reduce the response.
20. Explain why first exposure to an allergen produces less sever symptoms than subsequent exposure.
The initial exposure to the allergen results in B cells producing IgE antibodies. These antibodies attach to
mast cells. When the allergen is encountered again it reacts with the antibodies on the mast cells causing
them to release large amounts of histamines.
21. Do people die of AIDS? Explain in terms of the human immunodeficiency virus’ effect on the immune
system.
The AIDS virus (HIV) disarms T4 helper cells. Thus the immune response to all sorts of disease is reduced.
In the end it may be a relatively harmless disease, such as the flu, that kills an AIDS victim.
22. Distinguish between the following terms: pathogen, antigen, antibody and allergen.
A pathogen is a disease-causing agent whereas an antigen is a specific molecule recognised by the immune
system. Molecules on the pathogen’s surface may be antigens. An allergen is a molecule that provokes an
over-reaction of the immune system. An antibody is a molecule produced as a result of the presence of an
antigen.