AQA A Level Biology: Cell Recognition and the Immune System

Question 1

What is an antigen?

Answer 1

A foreign molecule (usually a protein) found on the surface of a pathogen that triggers an immune response.

Question 2

What is the role of antigens in the immune system?

Answer 2

Allow the immune system to identify:
* Pathogens
* Abnormal body cells
* Toxins
* Cells from other organisms (e.g., organ transplants)

Question 3

What are the two main types of immunity?

Answer 3

1. Non-specific immunity: Immediate, same for all pathogens. (phagocytes)
   
   2. Specific immunity: Slower but targets specific pathogens.
      (lymphocytes)

Question 4

What is the first line of defence in the immune system?

Answer 4

• Skin (physical barrier)
  
  • Mucus and cilia
  • Stomach acid
  • Lysozymes in tears and saliva

Question 5

What are phagocytes?

Answer 5

White blood cells that engulf and digest pathogens through phagocytosis.

Question 6

What are the steps of phagocytosis?

Answer 6

1. Recognition: Phagocyte binds to antigens on pathogen.
   
   2. Engulfment: Pathogen is engulfed by endocytosis into a phagosome.
   3. Lysosome fusion: Lysosomes fuse with phagosome, releasing digestive enzymes.
   4. Digestion: Pathogen is broken down. Waste released by exocytosis.
   5. Antigen presentation: Phagocyte presents antigens on its surface.

Question 7

What is an antigen-presenting cell (APC)?

Answer 7

A cell that displays foreign antigens on its surface after engulfing a pathogen, triggering the specific immune response.

Question 8

What are the two types of lymphocytes involved in the immune response?

Answer 8

1. T lymphocytes (T cells)
   
   2. B lymphocytes (B cells)

Question 9

What is the role of T lymphocytes?

Answer 9

Recognize antigens presented by APCs and stimulate other immune cells.

Question 10

What are the different types of T cells and their functions?

Answer 10

• Helper T cells (TH cells): Activate B cells and cytotoxic T cells.
  
  • Cytotoxic T cells (TC cells): Kill infected cells by releasing perforin.
  • Memory T cells: Provide long-term immunity.

Question 11

What is the role of B lymphocytes?

Answer 11

Produce antibodies in response to antigens.

Question 12

What are plasma cells?

Answer 12

Clones of activated B cells that produce large amounts of antibodies.

Question 13

What are memory B cells?

Answer 13

Cells that remain after infection and provide immunity by responding quickly if the same pathogen invades again.

Question 14

What is the structure of an antibody?

Answer 14

• 4 polypeptide chains (2 heavy, 2 light).
  
  • Variable region: Binds to specific antigens.
  • Constant region: Same in all antibodies, binds to immune cells.

Question 15

What are the functions of antibodies?

Answer 15

1. Agglutination: Clump pathogens together for easier engulfment.
   
   2. Neutralisation: Block toxins and prevent pathogen entry.
   3. Opsonisation: Mark pathogens for phagocytosis.

Question 16

What is the primary immune response?

Answer 16

• First exposure to an antigen.
  
  • Slow response as B cells need to divide and produce antibodies.

Question 17

What is the secondary immune response?

Answer 17

• Faster and stronger due to memory cells.
  
  • Antibodies produced more quickly to destroy the pathogen.

Question 18

What is the difference between active and passive immunity?

Answer 18

• Active immunity: Body produces its own antibodies (natural or vaccination).
  
  • Passive immunity: Antibodies are given directly (mother to baby or injection).

Question 19

What is natural active immunity?

Answer 19

Immunity gained after infection where memory cells provide future protection.

Question 20

What is artificial active immunity?

Answer 20

Immunity gained after vaccination with weakened or dead pathogens.

Question 21

What is natural passive immunity?

Answer 21

Antibodies passed from mother to baby through the placenta or breast milk.

Question 22

What is artificial passive immunity?

Answer 22

Antibodies injected into a person to provide immediate, short-term protection.

Question 23

What is a vaccine?

Answer 23

A preparation containing antigens that stimulate the immune system to produce memory cells.q

Question 24

What is herd immunity?

Answer 24

When a high percentage of the population is vaccinated, reducing the spread of the disease and protecting unvaccinated individuals.

Question 25

What is antigenic variation?

Answer 25

When pathogens change their surface antigens, making existing memory cells ineffective.

Question 26

What is the role of monoclonal antibodies?

Answer 26

* Targeted treatment: Bind to specific antigens. * Used in cancer treatment and pregnancy tests.

Question 27

What is the ELISA test?

Answer 27

A test that uses antibodies to detect the presence of specific antigens or antibodies in a sample.

Question 28

What are the ethical issues with vaccines?

Answer 28

* Animal testing. * Side effects. * Compulsory vaccination concerns.

Question 29

What is HIV and how does it affect the immune system?

Answer 29

HIV infects and destroys helper T cells, weakening the immune system and leading to AIDS.

Question 30

How does HIV replicate?

Answer 30

1. HIV binds to helper T cells. 2. RNA is converted to DNA using reverse transcriptase. 3. Viral DNA is inserted into host DNA. 4. New viruses are produced and released, killing T cells.