Immune Response

Question 1

Define the term antigen:

Answer 1

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
2. Often proteins on the surface of cells Note: proteins have a specific tertiary structure / shape allowing different proteins to act as specific antigens

Question 2

What are some non-self materials?

Answer 2

1. Pathogens (disease causing organisms) e.g. viruses, fungi, bacteria
2. Cells from other organisms of the same species e.g. organ transplant, blood transfusion
3. Abnormal body cells e.g. cancerous cells / tumours
4. Toxins released from bacteria

Question 3

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

Answer 3

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 4

Describe the cell mediated response (the response of T lymphocytes to a foreign antigen e.g. infected cells, cells of the same species):

Answer 4

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 5

Describe the humoral response (the response of B lymphocytes to a foreign antigen e.g. in blood/tissues):

Answer 5

1. 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)
2. Some become B plasma cells for the primary immune response – secrete large amounts of monoclonal antibody into blood
3. Some become B memory cells for the secondary immune response.

Question 6

Describe the Primary response – antigen enters body for the first time (role of plasma cells):

Answer 6

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

Question 7

Describe the secondary response – same antigen enters body again (role of memory cells):

Answer 7

1. Produces antibodies faster and at a higher concentration because
2. B and T memory cells present
3. B memory cells undergo mitosis quicker / quicker clonal selection