2.4 Cell recognition and the immune system

Question 1

What is an antigen?

Answer 1

● Foreign protein
● That stimulates an 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 (cell-surface membrane / cell wall) 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

Pathogens (disease causing microorganisms) eg. viruses, fungi, bacteria
Cells from other organisms of the same species (eg. organ transplants)
Abnormal body cells eg. tumour cells or virus-infected cells
Toxins (poisons) released by some bacteria

Question 4

Describe phagocytosis of pathogens (non-specific immune response

Answer 4

1 Phagocyte recognises foreign antigens on pathogen
2 Phagocyte engulfs pathogen by surrounding it with its cell membrane forming a phagosome vesicle
3 Pathogen contained in phagosome vesicle in cytoplasm
4 Lysosome fuses with phagosome and releases lysozymes (hydrolytic enzymes)
5 Lysozymes hydrolyse / digest pathogen

Question 5

Describe the response of T lymphocytes to a foreign antigen (the cellular
response)

Answer 5

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 (the humoral
response)

Answer 6

1. 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

What are antibodies?

Answer 7

● Quaternary structure proteins (4 polypeptide chains)
● Secreted by B lymphocytes eg. plasma cells in response to specific antigens
● Bind specifically to antigens forming antigen-antibody complexes

Question 8

Explain how antibodies lead to the destruction of pathogen

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

Question 9

Explain the differences between the primary & secondary immune response

Answer 9

● Primary- first exposure to antigen
○ Antibodies produced slowly & at a lower conc.
○ Takes time for specific B plasma cells to be
stimulated to produce specific antibodies
○ Memory cells produced
● Secondary- second exposure to antigen
○ Antibodies produced faster & at a higher conc.
○ B memory cells rapidly undergo mitosis to
produce many plasma cells which produce
specific antibodies

Question 10

Explain how vaccines provide protection to individuals against disease

Answer 10

Specific B lymphocyte with complementary receptor binds to antigen
Specific T helper cell binds to antigen-presenting cell and stimulates B cell
B lymphocyte divides by mitosis to form clones
Some differentiate into B plasma cells which release antibodies
Some differentiate into B memory cells
On secondary exposure to antigen, B memory cells rapidly divide by mitosis to produce B plasma cells
These release antibodies faster and at a higher concentration

Question 11

Explain how vaccines provide protections for populations against disease

Answer 11

● Herd immunity- large proportion of population vaccinated, reducing spread of pathogen
○ Large proportion of population immune so do not become ill from infection
○ Fewer infected people to pass pathogen on / unvaccinated people less likely to come in contact
with someone with disease

Question 12

What is a vaccine?

Answer 12

● Injection of antigens from dead or weakened pathogens
● Stimulating formation of memory cells

Question 13

Describe the differences between active and passive immunity

Answer 13

Active immunity:
-requires exposure to antigen
-memory cells involved
-takes longer to develop
-Long term immunity as antibody can be produce

Passive immunity:
-no exposure to antigen
-no memory cells involved
Faster acting
Short term immunity as
antibody hydrolysed (

Question 14

Explain the effect of antigen variability on disease and disease prevention

Answer 14

● Antigens on pathogens change shape / tertiary structure due to gene mutations (creating new strains)
● So no longer immune (from vaccine or prior infection)
○ B memory cell receptors cannot bind to / recognise changed antigen on secondary exposure
○ Specific antibodies not complementary / cannot bind to changed antigen