2.4 cell recognition and the immune system

Question 1

Antigen

Answer 1

Molecules which, when recognised as non-self, can trigger an immune response and lead to the production of antibodies

Often proteins on the surface of cells

Proteins have a specific tertiary structure allowing different proteins to act as specific antigens

Question 2

What the immune system identifies

Answer 2

Pathogens e.g. viruses, fungi, bacteria

Cells from other organisms of the same species e.g. organ transplant

Abnormal body cells

Toxins released from pathogens

Question 3

Phagocytosis of pathogens

Answer 3

Phagocyte recognises foreign antigens on the pathogen and binds to the antigen

Phagocyte engulfs pathogen by surrounding it with its cell surface membrane

Pathogen contained in vacuole in cytoplasm of phagocyte

Lysosome fuses with phagosome and releases lysozymes (hydrolytic enzymes) into the phagosome

These hydrolyse/digest the pathogen

Phagocyte becomes antigen presenting and stimulates specific immune response

Question 4

Cellular response (T lymphocytes)

Answer 4

T lymphocytes recognise antigen presenting cells after phagocytosis

Specific T helper cell with receptor complementary to specific antigen binds to it, becoming activated and dividing rapidly by mitosis to form clones which:

Stimulate B cells for the humoral response

Stimulate cytotoxic T cells to kill infected cells by producing perforin

Stimulate phagocytes to engulf pathogens by phagocytosis

Question 5

Humoral response

Answer 5

Clonal selection: in which specific B cell binds to APC and is stimulated by helper T cells which releases cytokines

Divides rapidly via mitosis to from clones

Some become B plasma cells for the primary immune response - secrete large amounts of monoclonal antibodies into the blood

Some become B memory cells for the secondary immune response

Question 6

Primary response

Answer 6

Produces antibodies slower and at a lower concentration

as not many B cells available that can make the required antibody &

T helpers need to activate B plasma cells to make the antibody

So infected individual will experience symptoms

Question 7

Secondary response

Answer 7

Produces antibodies at faster and higher concentration as

B and T memory cells present

B memory cells undergo mitosis quicker

Question 8

Describe and explain how the structure of an antibody relates to its function

Answer 8

Primary structure of protein = sequence of amino acids in a polypeptide chain

• determines the folds in the secondary structure as r groups interact
• determines the specific shape of the tertiary structure and position of hydrogen, ionic and disulphide bonds

Quaternary structure contains 4 polypeptide chains held together by H, I, D bonds

• enables the specific shaped variable region to form which is a complementary shape to specific antigen
• enables antigen-antibody complex to form

Question 9

How do antibodies work to destroy pathogens

Answer 9

Binds to 2 pathogens at same time, forming an antigen-antibody complex

Enables antibodies to clump pathogens together - agglutination

Phagocytes bind to the antibodies and phagocytise many pathogens at once

Question 10

Vaccination

Answer 10

Injection of antigens

From attenuated pathogens

Stimulates formation of memory cells

A vaccine can lead to symptoms because some of the pathogens may be live and reproduce and release toxins

Question 11

Use of vaccines to provide protection

Answer 11

Normal immune response but the memory cells are produced

On reinfection, the secondary response therefore produces antibodies faster and at higher concentration

Leading to the destruction of a pathogen/antigen before it can cause harm

Question 12

Herd immunity

Answer 12

Large proportion but not 100% vaccinated against a disease

Makes it more difficult for pathogen to spread as more people are immune so fewer in population carry the pathogen

Fewer susceptible so less likely that non-vaccinated will come in contact with disease

Question 13

Active immunity v Passive immunity

Answer 13

Initial exposure to antigen | no exposure to antigen

Memory cells involved | no memory cells involved

Antibody is produced & secreted by B plasma cells | antigen introduced into body from another organism

Slow, takes time to develop | fast acting

Long term immunity > antibody can be produced in response to a specific antigen again | short term immunity (antibody broken down)

Question 14

Ethical issues associated with the use of vaccines

Answer 14

Tested on animals before use on humans > animals have a CNS so feel pain

Tested on humans > volunteers may put themselves at unnecessary risk of contracting the disease because they think they’re fully protected

Can have side effects

Expensive - less money spent on research and treatments of diseases

Question 15

Antigen variability as explanation for why

Answer 15

New vaccines against a disease need to be developed more frequently e.g. influenza

Vaccines against a disease may be hard to develop or can’t be developed in the first place e.g. HIV

May experience a disease more than once e.g. common cold

Question 16

Explain the effect of antigen variability on disease

Answer 16

Change in antigen shape

Not recognised by B memory cells > no plasma cells

Not immune

Must re-undergo primary immune response > slower / releases lower concentration of antibodies

Disease symptoms felt

Question 17

Explain the effect of antigen variability on disease prevention

Answer 17

Change in antigen shape

Existing antibodies with specific shape unable to bind to changed antigens / form antigen-antibody complex

Immune system e.g memory cells wont recognise different antigens

Question 18

Evaluate methodology, evidence and data relating to the use of vaccinations

Answer 18

Successful vaccination programme:

• provides suitable vaccine: effective, no major side effects, low cost, easily produced
• provides herd immunity

Evaluating a conclusion from data: check correlation (could be due to another variable), repeatability (other studies?), validity (does it answer the question), potential bias

Question 19

Use of monoclonal antibodies

Answer 19

Monoclonal antibody = antibody produced from a single group of genetically identical B cells/plasma cells. Identical tertiary structure

Bind to specific complementary antigen

• have a binding site with a specific tertiary structure
• only 1 complementary antigen will fit

Question 20

Why are monoclonal antibodies useful in medicine

Answer 20

Only bind to specific target antigens

As

Antibodies have a specific tertiary structure that’s complementary to a specific antigen which can bind to the antibody

Question 21

Monoclonal antibodies & cancer cells

Answer 21

Monoclonal antibodies made to be complementary to antigens specific to cancer cells > cancer cells are abnormal body cells with different antigens

Anti-cancer drug attached to antibody

Antibody binds to cancer cells, forming antigen-antibody complex

Delivers attached anti-cancer drug directly to specific cancer cells so drug accumulates > fewer side effects

Question 22

Use your knowledge of monoclonal antibodies to suggest how this antibody stops the growth of a tumour

Answer 22

Antibody has a specific tertiary structure

Complementary shape to receptor protein

Prevents __ binding to receptor

Question 23

Monoclonal antibodies & pregnancy test

Answer 23

If pregnant , hCG binds to antibodies in application area = hCG-antibody complex

Travels up test strip, binds to antibodies at position 2 (blue line)

If not pregnant, no hCG in urine so hCG doesnt bind to antibodies in application area so doesnt bind to antibodies at position 2 = no blue line

Bind to antibodies at position 3 > blue line = control

Question 24

Use of antibodies in ELISA

Answer 24

Can determine if a patient has: antibodies to a certain antigen, antigen to a certain antibody (used to diagnose diseases or allergies)

Question 25

Why use control when performing ELISA

Answer 25

Controls enable a comparison with the test

To show that: only the enzyme and nothing else causes colour change, washing is effective and all unbound antibody is washed away

Question 26

Explain why the secondary and detection antibody must be washed away

Answer 26

Enzyme attached to antibody reacts with substrate turning the solution a different colour; indicates a positive test

Not washed out > enzymes will react with the substrate

Therefore give a positive result even if no antigen present

Question 27

Ethical issues associated with use of monoclonal antibodies

Answer 27

Animals are involved in the production of monoclonal antibodies

Although effective treatment for cancer & diabetes, has caused deaths when used in treatment of multiple sclerosis

Patients need to be informed of risk and benefits before treatment so they can make informed decisions

Question 28

Replication of HIV in helper T cells

Answer 28

HIV attachment proteins bind to CD4 receptor on the cell surface membrane of TH cells

Upon entry, HIV capsid uncoats and releases its contents (RNA, enzymes) into the cytoplasm

Reverse transcriptase uses viral RNA as a template to make complementary DNA strand. Eventually, double-stranded DNA is produced from the RNA

Enzyme integrate makes a cut into the host DNA, and integrates the viral DNA into the host

Viral DNA forces cell to make viral mRNA which forces the ribosomes to only make viral proteins, host then cannot survive.

Viral proteins are released

Viral particles infect more TH cells to replicate more HIV

Question 29

How HIV causes AIDS

Answer 29

Infects and kills T helper cells as it multiplies rapidly

• T helper cells then can’t stimulate cytotoxic T cells, B cells and phagocytes > impaired immune response

Immune system deteriorates

• more susceptible to disease
• diseases that wouldn’t cause serious problems in healthy immune system are deadly

Question 30

Why antibiotics are ineffective against viruses

Answer 30

Antibiotics cannot enter human cells, viruses are acellular

Viruses dont have own metabolic reactions

Question 31

HIV infection

Answer 31

HIV enters bloodstream

HIV binds to CD4 protein on T helper cell

Capsid fuses with cell membrane

HIV RNA and HIV enzymes enter the cell

reverse transcriptase converts viral RNA into DNA

New DNA inserted into cell nucleus

Host cell enzymes turn DNA into mRNA

MRNA leaves nucleus and translated into viral RNA and viral proteins by host cell ribosomes

New HIV particles assembled

HIV particles bud away from the cell taking a bit of the cell membrane as their lipid envelope

Question 32

Describe the role of antibodies in producing a positive result in an ELISA test

Answer 32

Antibody binds to antigen

Second antibody with enzyne attached is added & attaches to antigen

Solution added & colour changes

Question 33

Suggest how the immune response to this viral protein can result in the development of RA

Answer 33

Antibody against virus will bind to collagen

Results in destruction of collagen

Question 34

Suggest why they have not been considered safe

Answer 34

Inactive virus may become active

People may test HIV positive

Attenuated virus may become harmful

Question 35

How ELISA test is used

Answer 35

Sample is added to well in test plate

Plate is washed to remove unbound proteins

Add antibodies

Wash to remove unbound antibodies

Add solution