2.4 - Immune Response

Question 1

Define pathogen

Answer 1

A disease causing agent.

Question 2

Define antigen

Answer 2

A foreign protein or glycoprotein which stimulates an immune response

Question 3

Define antibody

Answer 3

A protein produced by B plasma cells which binds to a specific, complementary antigen

Question 4

1. Some white blood cells are phagocytic. Describe how these phagocytic white blood cells destroy bacteria

Answer 4

• Pathogen produces chemoattractant, which attract the phagocyte towards it
• The phagocyte engulfs the pathogen and holds it in a vesicle called a phagosome
• The phagosome membrane fuses with the membrane of lysosomes
• Lysosomes contain hydrolytic enzymes called lysozymes, which are released into the phagosome
• The lysozymes hydrolyse the pathogen
• Pathogen’s antigens are presented on the phagocyte cell surface membrane  phagocyte acts as an Antigen-Presenting Cell (APC)

Question 5

1. How do T cells stimulate an immune response?

Answer 5

● Phagocyte engulfs pathogen and presents the antigens on its cell surface membrane. It is now an Antigen-Presenting Cell (APC)
● T-helper cells with receptors complementary to the antigen bind to the APC
● T-helper cells become activated and divide by mitosis. They also secrete cytokines, which activate other cells
● T-helper cells:
i. Activate Cytotoxic T cells - Produce chemicals, which make the APC membrane permeable, which kills the infected cell
The holes also allow toxins to get in, which will definitely kill the cell and everything in it (e.g. viruses which have invaded)
ii. Become T-memory cells - Enable rapid response to future infections by the same pathogen
b. Activate specific B cells
c. Stimulate more phagocytosis by phagocytes

Question 6

What is the humoral immune response

Answer 6

The Humoral immune response involves clonal selection of B-lymphocytes, which produce antibodies that circulate in the blood and the lymph.

Question 7

1. Describe how B-lymphocytes respond when they are stimulated by antigens

Answer 7

● A specific B cell has protein receptors on its surface that are complementary to a specific antigen
● This specific B cell encounters the antigen on the surface of a pathogen or on the surface of an Antigen-Presenting Cell
● This specific B cell engulfs and presents the antigens on its own cell surface membrane
● The receptor of the activated T-helper cell binds to the complementary antigen presented on the cell surface membrane of the specific B-cell
● B cell is activated and divides by mitosis forming…
a. B plasma cells - Produce monoclonal antibodies, which circulate in the blood and bind to the complementary antigen on the pathogen. Ultimately resulting in the pathogens being killed by agglutination
b. B memory cells - remain in the blood, so if the pathogen re-invades, these cells divide by mitosis to make B plasma cells  secondary response is much faster

Question 8

1. What is Clonal selection of a B cell?

Answer 8

The activation of a B cell that has a specific protein receptor, which is complementary to a specific antigen.
The activated B cell divides by mitosis to produce genetically identical clones.
They then differentiate into B plasma cells and B memory cells.

Question 9

1. Why might one type of pathogen cause the activation of more than one type of B cell and more than one type of T cell?

Answer 9

• All cells (including pathogens) have more than 1 type of antigen on their surface.
• B cells can also be activated by toxins (act as antigens) produced by pathogens.

Question 10

Draw and label an antibody

Answer 10

Check notes

Question 11

What is a primary immune response?

Answer 11

A primary immune response occurs the first time an organism comes in contact with a specific pathogen (or antigen).
It is a relatively slow response because it takes time for;
o Clonal selection to occur to identify the T cells with the receptors complementary to the specific antigens present
o Clonal selection to occur to identify the B cells with the antibodies complementary to the specific antigens present
o B-cells to be activated (by T-helper cells) and divide by mitosis to produce B plasma cells
o B plasma cells to produce antibodies

Question 12

What is a secondary immune response?

Answer 12

● A secondary immune response occurs when an organism comes into contact with a specific pathogen, which it has come into contact with previously

Question 13

Why is the secondary immune response faster than the primary immune response?

Answer 13

● There are already lots of B memory cells in the blood
● The B memory cells divide by mitosis to make B plasma cells (without needing to be activated by T-helper cells)
● So, MORE antibodies are produced MORE QUICKLY

Question 14

What is antigenic variability? What are the consequences for an organism who encounters a pathogen with antigenic variability?

Answer 14

If a pathogen has antigenic variability, it means that the genes which code for the antigen proteins mutate from one generation to the next. Thus the antigens change from one generation to the next

The organism will not have B memory cells with antibodies complementary to the antigens
Therefore, the organism will have a new primary immune response and will experience symptoms of the disease

Question 15

Why do we only get chicken pox once but we can get the flu many times?

Answer 15

Chicken pox
1. The antigens are conserved from one generation to the next
2. Second infection is recognised by complementary receptors on B memory cells
3. The secondary immune response is fast/destroys the pathogen before symptoms are felt
Flu
1. Has antigenic variability/ antigens mutate from one generation to the next
2. Second infection is NOT recognised by complementary receptors on B memory cells
3. The primary immune response is slow/ symptoms are felt/ the person contracts the disease

Question 16

What is a vaccination?

Answer 16

• A vaccination contains dead or weakened pathogens, or antigens from them
• The vaccination stimulates the production of B plasma cells, antibodies and B memory cells

Question 17

Describe and explain how vaccination can lead to protection or people against bacterially caused diseases.

Answer 17

1. A vaccine is made from dead or weakened bacteria or antigens from them
2. B cells with complementary receptors bind to the antigen
3. Specific activated T-helper cells activate these B cells
4. B cells divide by mitosis to produce B plasma cells
5. The B plasma cells release antibodies, which are complementary to the bacterial antigens
6. Some B plasma cells become B memory cells
7. If the person is exposed again to the antigen or pathogen, the B memory cells become active and divide by mitosis to produce B plasma cells , which produce more antibodies more quickly.
8. These antibodies lead to the destruction of the pathogen before symptoms appear
9. Vaccinating a large proportion of the population results in the herd effect (which means that there are fewer people to pass on the disease, so unvaccinated people are protected)

Question 18

Define immunity

Answer 18

the ability of an organism to resist a particular infection (or toxin) by the action of the specific immune response.

Question 19

What is Herd Immunity?

Answer 19

● Herd Immunity is when a large enough proportion of the population is vaccinated, so that those who are not vaccinated are protected from the disease

Question 20

Why is Herd Immunity useful/ important?

Answer 20

● Herd immunity protects people in the population who cannot be vaccinated e.g. if they are very old, very young, have a weakened immune system, are taking immunosuppressants etc.

Question 21

What are the dangers of vaccines containing inactive form of virus

Answer 21

• Inactive virus may become active;
• Non-pathogenic virus may mutate and harm cells;
• Genetic information / protein may harm cells;
• People may test positive after vaccine used; Vaccinated people may develop disease from a different strain to that in the vaccine
• This may affect their work / life;

Question 22

What is active immunity? Give an example

Answer 22

Active immunity is when the antigens enter the body and the body produces its own T cells, B cells and antibodies. e.g. person infected with pathogen or a vaccine-containing pathogen is introduced to the body

Question 23

What is passive immunity? Give an example

Answer 23

Passive immunity is when antibodies are given to a person. e.g. from mother to child through breast milk, antibodies given via an injection, or anti-venom for snake bites

Question 24

Describe the difference between active and passive immunity

Answer 24

Active Immunity -
Memory Cells produced
Antibodies produced by plasma cells
Long term immunity as antibody is produced in response to antigen
Active immunity takes time to develop

Passive Immunity -
No memory cells produced
Antibodies introduced into body from outside
Short term as antibody that is introduced is broken down
Passive immunity is fast acting

Question 25

What are monoclonal antibodies?

Answer 25

* They are antibodies which are identical and come from one type of B plasma cell * They have the same tertiary structure and are all specific to only one antigen

Question 26

What is an ELISA Test?

Answer 26

Enzyme Linked ImmunoSorbent Assay This is when enzymes are attached to monoclonal antibodies to help with the detection of the presence of a specific molecule.

Question 27

Describe how monoclonal antibodies could be used to detect for the presence of antigens?

Answer 27

Antibodies complementary to the antigen are bound to the test plate 1. The sample containing antigens is added 2. Antigens bind to first antibody 3. The plate is washed 4. Second antibody is added. This antibody is complementary to the antigen AND has an enzyme attached to it 5. The second antibody attaches to antigen that is bound to the first antibody 6. The plate is washed again and any unbound second antibody is removed 7. A substrate is added, which binds to the enzyme and it will change colour if the antigen is present

Question 28

Explain why the plate is washed after adding the second antibody

Answer 28

To wash away all the unbound second antibodies If there is no antigen present, the second antibody with the enzyme will not bind to the antigen If the second antibody stays in the test plate, when the substrate is added, there will be a colour change even when the antigen is not present showing a FALSE positive result

Question 29

1) Explain why a psa test detects prostate cancer, but not any other disease.

Answer 29

* antibodies are specific and complementary to PSA * PSA is only associated with prostate cancer

Question 30

1) Explain why it is important to wash the well before the colourless substrate is added.

Answer 30

● Removes unbound second antibodies; ● Otherwise enzyme may be present and you may get colour change anyway. This will be a false positive result

Question 31

1) Explain why there will not be a colour change if the blood sample does not contain antigen

Answer 31

● The second antibody with the enzyme is washed away if no antigen is present ● There will be no colour change without the enzyme

Question 32

How are monoclonal antibodies used to treat cancer?

Answer 32

There are different ways they can be used to treat cancer – they all use the principle that the monoclonal antibodies are complementary to a receptor or antigen on the cancer cell. E.g. 1 ● The monoclonal antibody is complementary to a receptor protein on cancer cells ● In order for the cell to divide, a growth factor needs to bind to that receptor protein ● When the monoclonal antibody is present, it binds to the receptor, so the growth factor doesn’t find and the cell doesn’t divide E.g. 2 ● Use a monoclonal antibody that is complementary to an antigen on the cancer cell ● Bind a radioactive molecule to the monoclonal antibody ● The monoclonal antibody will bind to the cancer cell and thus take the radioactive molecule to the cell, which causes the cell to be killed

Question 33

Draw and label the diagram of a HIV

Answer 33

Check notes

Question 34

How does HIV replicate (10 marks)

Answer 34

1. The HIV’s attachment proteins binds to the CD4 receptor proteins on the surface of T-helper cell 2. The virus’s lipid envelope fuses with the cell membrane of the T-helper cell 3. The protein capsid breaks down 4. RNA and enzymes (e.g. reverse transcriptase) of the virus are now released into the cytoplasm of the host cell 5. Reverse transcriptase converts the viral RNA to viral DNA 6. The viral DNA is incorporated into the cell’s DNA 7. The viral DNA can now be transcribed into mRNA 8. Viral mRNA passes through the nuclear pore and attaches to a ribosome 9. Viral mRNA is translated into viral proteins that can be assembled into new HIV particles 10. HIV particles bud off the T-helper cell (so that the T-helper cell’s membrane forms the lipid envelope of the virus)