immunity

Question 1

antigen

Answer 1

foreign molecule
stimulates immune response leading to production of antibody

Question 2

How are cells identified by the immune system?

Answer 2

● Each type of cell has specific molecules on its surface that identify it
● Often proteins → have a specific tertiary structure

Question 3

What types of cells and molecules can the immune system identify?

Answer 3

1. Pathogens
2. Cells from other organisms of the same species
3. Abnormal body cells
4. Toxins

Question 4

phagocytosis

Answer 4

Phagocyte attracted by chemicals recognises (foreign) antigens on pathogen
2 Phagocyte engulfs pathogen by surrounding it with its cell membrane
3 Pathogen contained in vesicle / phagosome in cytoplasm of phagocyte
4 Lysosome fuses with phagosome and releases lysozymes
5 Lysozymes hydrolyse / digest pathogen

Question 5

Describe the response of T lymphocytes to a foreign antigen

Answer 5

T lymphocytes recognise (antigens on surface of) antigen presenting cells
Specific helper T cells with complementary receptors (on cell surface) bind to antigen on
antigen-presenting cell → activated and divide by mitosis to form clones which stimulate:
● Cytotoxic T cells → kill infected cells / tumour cells (by producing perforin)
● Specific B cells (humoral response - see below)
● Phagocytes → engulf pathogens by phagocytosis

Question 6

Describe the response of B lymphocytes to a foreign antigen

Answer 6

Clonal selection:
Specific B lymphocyte with complementary receptor (antibody on cell surface) binds to antigen
○ This is then stimulated by helper T cells (which releases cytokines)
○ So divides (rapidly) by mitosis to form clones
2. Some differentiate into B plasma cells → secrete large amounts of (monoclonal) antibody
3. Some differentiate into B memory cells → remain in blood for secondary immune response

Question 7

antibodies

Answer 7

Quaternary structure proteins (4 polypeptide chains)
Secreted by B lymphocytes e
Bind specifically to antigens forming antigen-antibody complexes

Question 8

Explain how antibodies lead to the destruction of pathogens

Answer 8

Antibodies bind to antigens on pathogens forming an antigen-antibody complex
○ Specific tertiary structure so binding site / variable region binds to complementary antigen
● Each antibody binds to 2 pathogens at a time causing agglutination (clumping) of pathogens
● Antibodies attract phagocytes
● Phagocytes bind to the antibodies and phagocytose many pathogens at once