Immune response

Question 1

Define the term antigen

Answer 1

Molecules which, when recognised as non-self/foreign by the immune system, can stimulate an immune response and lead to the production of antibodies

Often proteins on the surface of cells

Question 2

Describe the non-specific immune response (Phagocytosis of pathogens)

Answer 2

1)Phagocyte e.g. macrophage recognises foreign antigens on the pathogen and binds to the antigen
2)Phagocyte engulfs pathogen by surrounding it with its cell surface membrane /cytoplasm
3)Pathogen contained in vacuole/vesicle/phagosome in cytoplasm of phagocyte
4)Lysosome fuses with phagosome and releases lysozymes (hydrolytic enzymes) into the phagosome
5)These hydrolyse / digest the pathogen
6)Phagocyte becomes antigen presenting and stimulates specific immune response

Question 3

Describe the cellular response

Answer 3

1)T lymphocytes recognises antigen presenting cells after phagocytosis (foreign antigen)

2)Specific T helper cell with receptor complementary to specific antigen binds to it, becoming activated and dividing rapidly by mitosis to form clones which:
a) Stimulate B cells for the humoral response
b) Stimulate cytotoxic T cells to kill infected cells by producing perforin
c) Stimulate phagocytes to engulf pathogens by phagocytosis

Question 4

Describe the humoral response

Answer 4

Clonal selection:
a) Specific B cell binds to antigen presenting cell and is stimulated by helper T cells which releases cytokines
b) Divides rapidly by mitosis to form clones (clonal expansion)

Some become B plasma cells for the primary immune response – secrete large amounts of monoclonal antibody into blood

Some become B memory cells for the secondary immune response

Question 5

Describe the Primary response
(role of plasma cells)

Answer 5

• Produces antibodies slower and at a lower concentration because
• Not many B cells available that can make the required antibody
• T helpers need to activate B plasma cells to make the antibodies (takes
  time)
• So infected individual will express symptoms

Question 6

Describe the secondary response
(role of memory cells)

Answer 6

Produces antibodies faster and at
a higher concentration because

• B and T memory cells present
• B memory cells undergo mitosis
  quicker / quicker clonal selection

Question 7

What are antibodies?

Answer 7

Quaternary structured protein
Secreted by B lymphocytes
Binds specifically to antigens (monoclonal) forming an antigen-antibody complex

Question 8

Describe and explain how the structure of an
antibody relates to its function

(refer back to a* booklet for diagram)

Answer 8

Primary structure of protein = sequence of amino acids in
a polypeptide chain

• Determines the folds in the secondary structure as R
  groups interact
• Determines the specific shape of the tertiary structure
  and position of hydrogen, ionic and disulfide bonds
• Quaternary structure is comprised of 4 polypeptide
  chains (tertiary structured) held together by hydrogen,
  ionic and disulfide bonds
• Enables the specific shaped variable region (binding site)
  to form which is a complementary shape to a specific
  antigen
• Enables antigen-antibody complex to form

Question 9

How do antibodies work to destroy pathogens

Answer 9

Binds to two pathogens at a time (at variable region/binding site)
forming an antigen-antibody complex

• Enables antibodies to clump the pathogens together – agglutination
• Phagocytes bind to the antibodies and phagocytose many pathogens
  at once
• Note: the hinge region means an antibody can bind to antigens /
  pathogens different distances part