3.2.4 Cell recognition and the immune system

Question 1

What are antigens?

Answer 1

Molecules (usually proteins) that can generate an immune response when detected by the body. They’re usually found on the surface of cells.

Question 2

What does the humoral response involve?

Answer 2

B-cells, clonal selection and the production of monoclonal antibodies form the humoral response.

Question 3

What does the cellular response involve?

Answer 3

The T-cells and phagocytes form the cellular response.

Question 4

Describe the function of antibodies.

[2 marks]

Answer 4

• Antibodies bind pathogens together (agglutinate pathogens).
• This allows phagocytes to engulf many pathogens at once.

Question 5

What type of cell does HIV replicate in?

Answer 5

HIV replicates inside its host’s helper T-cells.

Question 6

Why can’t antibiotics be used to treat HIV?

Answer 6

• Antibiotics are designed to only target the bacterial enzymes and ribosomes.
• But HIV uses the human enzymes and ribosomes to replicate…
• …so antibiotics can’t inhibit them because they don’t target human processes.

Question 7

HIV is the virus that eventually causes AIDS. Describe the structure of HIV.
[4 marks]

Answer 7

• HIV has a core that contains the genetic material (RNA) and some proteins.
• It has an outer layer called the capsid, which is made of protein,…
• …surrounded by an envelope that is made from the membrane of the host cell.
• There are attachment proteins sticking out from the envelope.

Question 8

Describe and explain how a secondary immune response differs to a primary immune system.
[4 marks]

Answer 8

• A secondary immune response is a faster and stronger response than the primary response.
• This is because memory cells are produced during the primary response, which are able to recognise the foreign antigen when it is encountered again.
• During the second infection, memory B-cells can quickly divide to form plasma cells, which secrete the correct antibody to the antigen.
• Memory T-cells quickly divide into the right type of T-cells to kill the cell carrying the antigen.

Question 9

How do vaccines cause immunity?

Answer 9

• Vaccines contain antigens that cause your body to produce memory cells against a particular pathogen.
• When you’re affected the second time with the same pathogen they activate the secondary response.

Question 10

Vaccines can be used to protect people against some diseases. Not all individuals in a population must receive the vaccine for a vaccination programme to be successful. Explain why this is the case.
[3 marks]

Answer 10

• When some individuals in a population receive the vaccine, the occurrence of the disease in the population is reduced.
• This means that those in the population who haven’t been vaccinated are less likely to become infected.
• This is called herd immunity.

Question 11

The influenza virus causes the flu. Explain why it is possible to suffer from the flu more than once.
[4 marks]

Answer 11

• The flu virus is able to change its surface antigens/shows antigenic variation.
• This means that when you’re infected for a second time with a different strain, the memory cells produced from the first infection will not recognise the new/different antigens.
• The immune system has to carry out a primary response against these new antigens.
• This takes time and means you become ill.

Question 12

Explain what antigenic variation is.

Answer 12

A formation of different antigens due to changes in the genes of a pathogen.

Question 13

Immunity from a disease can be either active or passive.
Explain why active immunity offers long-term protection against a disease, whereas passive immunity only offers protection in the short-term.
[2 marks]

Answer 13

• Active immunity involves the production of memory cells specific to a particular antigen. This means the immune system is able to mount a secondary immune response if the same antigen is detected again.
• Passive immunity only offers short-term protection because the antibodies given are broken down in the body.
  OR
• Memory cells are not produced, so the body can’t mount a secondary immune response.

Question 14

Immunity from a disease can be either active or passive.
It normally takes 14 days for immunity to develop after receiving a vaccine. Explain why vaccines do not usually offer immediate protection against a disease.
[1 mark]

Answer 14

It takes time for the body to produce antibodies/memory cells against the antigens in the vaccine.

Question 15

What is active immunity?

Answer 15

A type of immunity you get when your immune system makes its own antibodies after being stimulated by an antigen.

Question 16

What is passive immunity?

Answer 16

A type of immunity you get from being given antibodies made by a different organism - your immune system doesn’t produce any antibodies of its own.

Question 17

NMO is a disease that leads to damage to nerve cells in the spinal cord. A person with NMO produces an anti-AQP4 antibody that attacks only these nerve cells.
Explain why the anti-AQP4 antibody only damages these cells.
[4 marks]

Answer 17

• (Anti-AQP4) antibody has a (specific) tertiary structure.
• Has binding site/variable region that only binds to one antigen.
• Antigen to this antibody (only) found on these nerve cells.
• So, antibody (only) binds to these nerve cells (causing damage).

Question 18

A new treatment for NMO involves using a monoclonal antibody. The structure of the variable region of this monoclonal antibody is identical to the variable region of an anti-AQP4 antibody, but the rest of its structure is different.
Suggest how this monoclonal antibody prevents anti-AQP4 damaging nerve cells.
[2 marks]

Answer 18

• The monoclonal antibody binds to nerve cell antigen so less/no anti-AQP4 can bind.
• When a monoclonal antibody binds it doesn’t cause damage to nerve cell.

Question 19

What are monoclonal antibodies?

Answer 19

Antibodies produced from a single group of genetically identical B-cells (plasma cells).

Question 20

Describe how monoclonal antibodies can be used to target a drug to cancer cells.
[4 marks]

Answer 20

• Monoclonal antibodies are made against antigens specific to cancer cells/tumours markers.
• An anti-cancer drug is attached to the antibodies.
• The antibodies bind to the antigens/tumour markers on cancer cells because their binding sites have a complementary shape.
• This delivers the anti-cancer drug to the cells.

Question 21

What are the two non-specific defence mechanisms?

Answer 21

• Physical barriers (e.g., skin)

- Phagocytosis

Question 22

What are the two specific defence mechanisms?

Answer 22

• The cell-mediated response (T lymphocytes)

- The humoral response (B lymphocytes)

Question 23

State two differences between a specific and non-specific defence mechanism.

Answer 23

• A specific mechanism distinguishes between different pathogens but responds more slowly than a non-specific mechanism.
• A non-specific mechanism treats all pathogens in the same way but responds more rapidly than a non-specific mechanism.

Question 24

After a pathogen gains entry to the body, it is often a number of days before the body’s immune system begins to control it.
Suggest a possible reason why this is so.

Answer 24

The lymphocytes that will finally control the pathogen need to build up their numbers which takes time.

Question 25

After a pathogen gains entry to the body, it is often a number of days before the body’s immune system begins to control it.
Suggest why it would be inaccurate to say that the body takes days to ‘respond’ to the pathogen.

Answer 25

The body responds immediately by ‘recognising’ the pathogen (and by phagocytosis); the delay is in building up numbers of lymphocytes and therefore controlling the pathogen.

Question 26

Pathogens that invade the body may be engulfed by cells which carry out __________. The engulfed pathogen forms a vesicle known as a _________. Once engulfed the pathogen is broken down by enzymes called _______ released from organelles called ________.

Answer 26

Pathogens that invade the body may be engulfed by cells which carry out phagocytosis. The engulfed pathogen forms a vesicle known as a phagosome. Once engulfed the pathogen is broken down by enzymes called lysozyme released from organelles called lysosomes.

Question 27

Among other places, lysozymes are found in tears. Suggest a reason why this is so.

Answer 27

• The protective covering of the eye, and especially the tear ducts, are potential entry points for pathogens. - The eyes are vulnerable to infection because the coverings are thin to allow light through.
• Lysozyme will break down the cell walls of any bacterial pathogens and so destroy them before they can cause harm.

Question 28

State two similarities between T cells and B cells.

Answer 28

Any two from:

• Both are types of white blood cell.
• Both have a role in immunity.
• Both are produced from stem cells.

Question 29

State two differences between T cells and B cells.

Answer 29

• T cells mature in the thymus gland while B cells mature in the bone marrow.
• T cells are involved in cell-mediated immunity while B cells are involved in humoral immunity.