2.4 Cell recognition and the immune system pt 1

Question 1

What is an antigen?

Answer 1

● Foreign molecule / protein / glycoprotein / glycolipid
● 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 (or glycoproteins / glycolipids)

Question 3

What types of cells and molecules can the immune system identify? (4)

Answer 3

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

Question 4

Describe phagocytosis of pathogens (non-specific immune response) (5)

Answer 4

1. Phagocyte attracted by chemicals / recognises (foreign) antigens on pathogen
2. Phagocyte engulfs pathogen by surrounding it with its cell membrane
3. Pathogen contained in phagosome in cytoplasm of phagocyte
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) (4)

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) (3)

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

Extra About B and T lymphocytes

Answer 7

B lymphocytes can recognise free antigens eg. in blood or tissues, not just antigen presenting cells.

T lymphocytes recognise (antigens on surface of) antigen presenting cells eg. infected cells, phagocytes
presenting antigens, transplanted cells, tumour cells etc.

Question 8

What are antibodies? (3)

Answer 8

● 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 9

Describe the structure of an antibody (7)

Answer 9

Heavy polypeptide chain
Constant region
Hinge region
Light polypeptide chain
Variable region
Antigen binding site
Antigen
Disulfide bridges

Question 10

Explain how antibodies lead to the destruction of pathogens (4)

Answer 10

● 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 11

Explain the differences between the primary & secondary immune response

Answer 11

● 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 12

What is a vaccine?

Answer 12

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

Question 13

Explain how vaccines provide protection to individuals against disease (7)

Answer 13

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

Question 14

Explain how vaccines provide protections for populations against disease

Herd immunity

Answer 14

● 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 15

Describe the differences between active and passive immunity (5)

Answer 15

• In Active immunity, Initial exposure to antigen eg. vaccine or primary infection whereas in passive immunity there is no exposure to antigen
• In active immunity, Memory cells involved whereas in passive immunity there are no memory cells involved.
• In active immunity Antibody produced and secreted by B plasma cells Whereas in Passive immunity the Antibody is introduced from another organism eg. breast milk / across placenta from mother
• In active immunity it is Slow; takes longer to develop Whereas in passive immunity it is Faster acting
• In active immunity it is a Long term immunity as antibody can be produced in response to a specific antigen again Whereas in passive it is Short term immunity as
  antibody hydrolysed (endo/exo/dipeptidases)

Question 16

Explain the effect of antigen variability on disease and disease prevention (2)

Answer 16

● 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

Question 17

(Example applications: yearly new flu vaccines developed, no vaccine for HIV, can catch a cold many times)

Answer 17

se

Question 18

Describe how phagocytosis of a virus leads to presentation of its antigens

Answer 18

1. Phagosome / vesicle fuses with lysosome;
2. (Virus) destroyed by lysozymes / hydrolytic enzymes;
3. Peptides / antigen (from virus) are displayed on the cell
   membrane;