Topic 2C: Cells and the Immune System

Question 1

What is a pathogen? [1]

Answer 1

A microorganism (e.g. bacteria, fungi, protists) or viruses that cause infectious disease.

Question 2

What is a toxin?

Answer 2

A poisonous substance produced by living organisms

Question 3

What is an antigen?

Answer 3

A foreign in molecule (usually a protein) that stimulates an immune response

Question 4

When a pathogen enters the body it may be destroyed by phagocytosis.
Describe how. [4]

Answer 4

1. Phagocyte attracted by a substance / recognises (foreign) antigen;
2. (Pathogen) engulfed;
3. Enclosed in vesicle/phagosome;
4. (Phagosome) fuses with lysosome;
5. Lysosome contains lysozymes;
6. Pathogen digested (by lysozymes).

Question 5

What is an antibody?

Answer 5

A protein capable of binding a specific antigen

Question 6

Why do antibodies have a quaternary structure? [1]

Answer 6

1. More than one / four polypeptide chains

Question 7

Explain why an antibody binds to a specific antigen. [3]

Answer 7

1. Antibody / variable region has specific amino acid sequence / primary structure;
2. The shape / tertiary structure of the binding site is complementary to / binds with these antigens;
3. Forms complex between antigen and antibody.

Question 8

Describe how a phagocyte destroys a pathogen present in the blood. [3]

Answer 8

1. Engulfs;
2. Forming phagosome and fuses with lysosome;
3. Enzymes digest.

Question 9

Give two types of cell, other than pathogens, that can stimulate an immune response. [2]

Answer 9

1. (Cells from) other organisms/transplant;
2. Abnormal/cancer/tumour (cells);
3. (Cells) infected by virus.

Question 10

What is the role of disulphide bridges in forming the quaternary structure of an antibody? [1]

Answer 10

1. Joins (different) polypeptides

Question 11

Describe how phagocytosis of a virus leads to presentation of its antigens. [3]

Answer 11

1. Phagosome fuses with lysosome;
2. (Virus) destroyed by lysozymes;
3. Antigen (from virus) displayed in the cell membrane

Question 12

Describe how presentation of a virus antigen leads to the secretion of an antibody against this virus antigen. [3]

Answer 12

1. Helper T cell binds to the antigen (on the antigen-presenting cell/phagocyte);
2. This helper T cell stimulates a specific B cell;
3. B cell divides by mitosis;
4. (Forms) plasma cells that release antibodies.

Question 13

What are monoclonal antibodies?

Answer 13

Antibodies with the same tertiary structure produced from a genetically identical set of plasma cells.

Question 14

How is immunity achieved?

Answer 14

Through the presence of antibodies against the antigens of that pathogen in a person’s body.

Question 15

How does a vaccine make someone immune to a pathogen? [5]

Answer 15

1. (Vaccine contains) antigen / attenuated/dead/inactive pathogen;
2. (Specific) helper T cell stimulates B cell specific to antigen;
3. B cell clones/divides by mitosis;
4. Plasma cells release antibodies;
5. Memory cells produced meaning higher concentration of antibodies/antibodies produced faster in secondary response/on infection with the actual pathogen.

Question 16

What is herd immunity?

Answer 16

A form of indirect protection from infectious disease that occurs when a sufficient percentage of a population has become immune to an infection, reducing the likelihood of infection spreading to individuals who lack immunity.

Question 17

Describe how B lymphocytes would respond to a vaccination against a virus.

Do not include details of the cellular response in your answer. [3]

Answer 17

1. B cell binds to specific/complementary (viral) antigen;
2. B cell divides by mitosis;
3. (B cells develop into) plasma cells that release antibodies (against the virus);
4. (B cells develop into) memory cells

Question 18

What is an allergic reaction?

Answer 18

An immune response to an otherwise harmless substance.

Question 19

Explain how treatment with antivenom works and why it is essential to use passive immunity, rather than active immunity. [2]

Answer 19

1. (Antivenom) antibodies bind to the toxin/venom/antigen and (causes) its destruction;
2. Active immunity would be too slow.

Question 20

When making an antivenom, a mixture of venoms from several snakes of the same species is used.

Suggest why. [2]

Answer 20

1. May be different form of antigen/toxin (within one species);
2. Different antibodies (needed in the antivenom)

Question 21

During the production of antivenom, the animals used are under ongoing observation by a vet.

Suggest one reason why. [1]

Answer 21

1. (So) the animal does not suffer from the venom/vaccine/toxine;
2. (So) the animal does not suffer anaemia/does not suffer as a result of blood collection;
3. (So) the animal does not have pathogen that could be transferred to humans.

Question 22

During vaccination, each animal is initially injected with a small volume of venom. Two weeks later, it is injected with a large volume of venom.

Use your knowledge of the hum oral immune response to explain this vaccination programme. [3]

Answer 22

1. B cells specific to the venom reproduce by mitosis;
2. (B cells produce) plasma cells and memory cells;
3. The second dose produces antibodies (in secondary immune response) in higher concentration and quickly

Question 23

NMO is a disease that leads to damage to nerve cells in the spinal cord. A person with NMO produces anti-AQP4 antibody that attacks only these nerve cells.

Explain why the anti-AQP4 antibody only damages these cells. [4]

Answer 23

1. (Anti-AQP4) antibody has a (specific) tertiary structure;
2. Has binding site / variable region that only binds/complementary to one antigen;
3. Antigen to this antibody (only) found in these nerve cells;
4. So, antibody (only) binds to / forms antigen-antibody complex with these nerve cells (causing damage).