2.4 Cell Recognition And The Immune System

Question 1

What is an antigen? (2)

Answer 1

• foreign molecule / protein / glycoprotein / glycolipid
• that stimulates an immune response leading to the production of an antibody

Question 2

How are cell identified by the immune system? (2)

Answer 2

• each type of cell has specific molecules on its surface (cell membrane /cell wall) that identifies it
• often proteins - have a specific tertiary structure (or glycoproteins / glycolipids)

Question 3

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

Answer 3

• pathogens (disease causing microorganisms) e.g. viruses, fungi, bacteria
• cells from other organisms of the same species e.g. organ transplants
• abnormal body cells e.g. tumour cells or virus - infected cells
• 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 vesicles/ 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) (5)

Answer 5

• T lymphocytes recognise antigens on surface of antigen presenting cells e.g. infected cells, phagocytes, transplanted cells etc
• specific T helper cells with complementary receptors on cell surface bind to antigen on APC - 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)
• phagocytes - engulf pathogens by phagocytosis

Question 6

Des robe the response of B lymphocytes to a foreign antigen (the humoral response) (5)

Answer 6

Clonal selection:
- specific B lymphocytes 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 from clones
- some differentiates int B plasma cells - secrete large amount of monoclonal antibody
- some differentiate into B memory cells - remain in blood for secondary immune response

Question 7

What are antibodies? (3)

Answer 7

• quaternary structure proteins (4 polypeptide chains)
• secreted by B lymphocytes e.g. plasma cells in response to specific antigens
• bind specifically to antigens forming antigen - antibody complexes

Question 8

Describe the structure of of an antibody (8)

Answer 8

1. Antigen = top of antibody, complementary
2. Antigen binding site = where the antigen and antibody bind
3. Variable region = top part of antibody which changes depending on the antibody
4. Constant region = bottom part which is always the same
5. Disulfide bridges = bond between the two constant regions
6. Light polypeptide chain = between variable and constant region
7. Hinge region = area that moves / bends
8. Heavy polypeptide chain = between two constant regions

Question 9

Explain how antibodies lead to the destruction of pathogens (5)

Answer 9

• antibodies bind to antigen on pathogens forming an antigen - antibody complex
• specific tertiary structure so binding site / variable region binds to complementary antigen
• each Antony binds to two pathogens at a time causing agglutination (clumping) of pathogens
• antibodies attract phagocytes
• phagocytes bind to the antibodies and phagocytose many pathogens at once

Question 10

Explain the differences between the primary & secondary immune response (7)

Answer 10

• 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 = seconds 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 11

What is a vaccine? (2)

Answer 11

• injection is antigens from attenuated (dead or weakened) pathogens
• stimulating formation of memory cells produced

Question 12

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

Answer 12

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 plasma B 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 13

Explain how vaccines provide protections for populations against disease (3)

Answer 13

• 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 14

Describe the differences between active and passive immunity (5 differences)

Answer 14

1. Initial exposure to antigen e.g. vaccine or primary infection ~ no exposure to antigen
2. Memory cells involved ~ no memory cells involved
3. Antibody produced and secreted by B plasma cells ~ antibody introduced from another organism e.g. breast milk / across placenta from mother
4. Slow; takes longer to develop ~ faster acting
5. Long term immunity as antibody can be produced in espouse to a specific antigen again ~ short term immunity as antibody hydrolysed

Question 15

Explain the effect of antigen variability on disease and disease prevention (4)

Answer 15

• antigens on pathogens change shape / tertiary structure due to gene mutation (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 are not complementary/ cannot bind to changed antigen on secondary

Question 16

Describe the structure of a HIV particle ( 5 things)

Answer 16

• outside is attachment protein
• membrane is a lipid envelope
• RNA and reverse transcriptase enzyme surrounded by capsid

Question 17

Describe the replication of HIV in helper T cells (7)

Answer 17

1. HIV attachement proteins attach to receptors on helper T cell
2. Lipid envelope fuses with cell membrane, releasing capsid into cell
3. Capsid uncoats, releasing RNA and reverse transcriptase
4. Reverse transcriptase converts viral RNA to DNA
5. Viral DNA inserted / incorporated into helper T cell DNA (may remain latent)
6. Viral protein / capsid / enzymes are produced
   a. DNA transcribed into HIV mRNA
   b. HIV mRNA translated into new HIV proteins
7. Virus particles assembled and released from cell (via budding)

Question 18

Explain how HIV cause the symptoms of acquired immune deficiency syndrome (AIDS) (5)

Answer 18

• HIV infect and kills T helper cells (host cell) as it multiplies rapidly
• so T helper cells can’t stimulate cytotoxic T cells, B cells and phagocytes
• so B plasma cells can’t release as many antibodies for agglutination & destruction of pathogens
• Immune system deteriorates - more susceptible to opportunistic infections
• pathogens reproduce, release toxins and damage cells

Question 19

Explain why antibiotics are ineffective against viruses (2)

Answer 19

Viruses do not have structures / processes that antibiotics inhibit:
- viruses do not have metabolic processes e.g. do bot make protein / ribosomes
- viruses do not have bacterial enzymes / murein cell wall

Question 20

What is a monoclonal antibody (2)

Answer 20

• antibody produced from genetically identical / clones B lymphocyte / plasma cell
• so have same tertiary structure

Question 21

Explain how monoclonal antibodies can be used in medical treatments (4)

Answer 21

• monoclonal antibody has a specific tertiary structure / binding site / variable region
• complementary to receptor / protein / antigen found only on a specific cell type e.g. cancer cell
• therapeutic drug attached to antibody
• antibody binds to specific cell, forming antigen - antibody complex, delivering drug

Question 22

Explain how monoclonal antibodies can be used in medical diagnosis (4)

Answer 22

• monoclonal antibody has a specific tertiary structure / binding site / variable region
• complementary to specific receptor / protein / antigen associated with diagnosis
• dye / stain / fluorescent marker attached to antibody
• antibody binds to receptor / protein / antigen, forming antigen - antibody complex

Question 23

Explain the use of antibodies in the ELISA test to detect antigens - direct ELISA (4)

Answer 23

1. Attach sample with potential antigens to well
2. Add complementary monoclonal antibodies with enzymes attached - bind to antigens if present
3. Wash well - remove unbound antibodies to prevent false positive
4. Add substrate - enzymes create products that cause a colour change positive result

Question 24

Explain the use of antibodies in the ELISA test to detect antigens - sandwich ELISA (5)

Answer 24

1. Attach specific monoclonal antibodies to well
2. Add sample with potential antigens, then wash well
3. Add complementary monoclonal antibodies with enzymes attached - bind to antigens if present
4. Wash well - remover unbound antibodies to prevent false positive
5. Add substrate - enzymes create products that cause a colour change positive result

Question 25

Explain the use of antibodies in the ELISA test to detect antibodies indirect elisa (5)

Answer 25

1. Attach specific antigens to well
2. Add sample with potential antibodies, wash well
3. Add complementary monoclonal antibodies with enzymes attached - bind to antibodies if present
4. Wash well - remover unbound unbound antibodies
5. Add substrate - enzymes create products that cause a colour change positive result

Question 26

Suggest the purpose of a control well in the ELISA test (2)

Answer 26

• compare to test to show only enzyme causes colour change
• compare to test to show all unbound antibodies have been washed away

Question 27

Suggest why failur to thoroughly was the well can result in a false positive test (2)

Answer 27

• antibody with enzymes attached remains / not washed out
• so substrate converted into colour product

Question 28

Discuss some general ethical issues associated with the use of vaccines and monoclonal antibodies (5)

Answer 28

• pre-clinical testing on / use of animals - potential stress / harm / mistreatment
• but animal is not killed & helps produce new drugs to reduce human suffering
• clinical trials on humans - potential harm / side-effects
• vaccines - may continue high risk activités and still develop / pass on pathogen
• use of drug - potentially dangerous side effects

Question 29

Suggest some points to consider when evaluating methodology repeating to the use of vaccines and monoclonal antibodies (5)

Answer 29

• was the sample size large enough to be representative?
• were participants diverse in terms of age, sex, ethnicity, and health status?
• were placebo / control groups used for comparison?
• was the duration of the study long enough to show long-term effects?
• was the trial double-blind (neither the doctor / patient knew who was given the drug / placebo to reduce bias?

Question 30

Suggest some points to consider when evaluating evidence and data relating to the use of vaccines and monoclonal antibodies (6)

Answer 30

• what side effects were observed and how frequently did they occur?
• was a statistical test used to see if there was a significant difference between start & final results?
• was the standard deviation of final results large, showing some people did not benefit?
• did standard deviation of start & final results overlap, showing there may not be a significant difference?
• what dosage was optimum? Does increasing the doe increase effectiveness enough to justify extra cost?
• was the cost of production & distribution low enough?