3.2.4 Cell recognition and the immune system

Question 1

Define antigen

Answer 1

• Molecules which, when recognised as non-self/foreign by the immune system, can stimulate an
  immune response and lead to the production of antibodies
• Often proteins/glycoproteins on the surface of cells

Question 2

Why are antigens so specific what do they allow the immune system to identify ?

Answer 2

• Pathogens (disease causing organisms) e.g. viruses, fungi, bacteria
• Cells from other organisms of the same species e.g. organ transplant, blood transfusion
• Abnormal body cells e.g. cancerous cells / tumours
• Toxins released from bacteria

Question 3

Describe the process of phagocytosis (Non-specific immune response )

Answer 3

1. Phagocyte e.g. macrophage recognises foreign antigens on the pathogen and binds to
   the antigen
2. Phagocyte engulfs pathogen by surrounding it with its cell surface membrane / cytoplasm
3. Pathogen contained in vacuole/vesicle called a phagosome in cytoplasm of phagocyte
4. Lysosome fuses with phagosome (phagolysosome) and releases lysozymes (hydrolytic
   enzymes) into the phagosome
5. These hydrolyse / digest the pathogen
6. Phagocyte becomes antigen presenting (APC) and stimulates specific immune response

Question 4

Describe the cellular response (T-lymphocytes to a foreign antigen e.g. infected cells )

Answer 4

1. T lymphocytes recognises antigen presenting cells after phagocytosis (foreign antigen)
2. Specific T helper cell with receptor complementary to specific antigen binds to it, becoming
   activated and dividing rapidly by mitosis to form clones which:
   a) Stimulate B cells for the humoral response
   b) Stimulate cytotoxic T cells to kill infected cells by producing perforin
   c) Stimulate phagocytes to engulf pathogens by phagocytosis

Question 5

Describe the humoral response (the response of B lymphocytes to a foreign antigen
e.g. in blood/tissues)

Answer 5

1. Clonal selection:
   a) Specific B cell binds to antigen presenting cell and is stimulated by helper T cells which
   releases cytokines
   b) Divides rapidly by mitosis to form clones (clonal expansion)
2. Some become B plasma cells for the primary immune response – secrete large amounts of
   monoclonal antibody into blood
3. Some become B memory cells for the secondary immune response

Question 6

Describe the primary response – antigen enters body
for the first time (role of plasma cells)

Answer 6

• Produces antibodies slower and at a lower
  concentration because
• Not many B cells available that can make
  the required antibody
• T helpers need to activate B plasma cells
  to make the antibodies (takes time)
• So infected individual will express symptoms

Question 7

Describe the secondary response ( same antigen enters body again(role of memory cells )

Answer 7

• Produces antibodies faster and at a higher
  concentration because
• B and T memory cells present
• B memory cells undergo mitosis quicker / quicker
  clonal selection

Question 8

Define antibodies and describe how antibodies work

Answer 8

• Quaternary structured protein (immunoglobin)
• Secreted by B lymphocytes e.g. plasma cells and produced
  in response to a specific antigen
• Binds specifically to antigens (monoclonal) forming an
  antigen-antibody complex
• two binding sites so one antibody can bind to two
  pathogens at a time (at variable region/binding site)
  forming two separate antigen-antibody complex
• Enables antibodies to clump the pathogens together –
  agglutination = easier for engulfing
  *the hinge regions aid binding to antigens

Question 9

What is a vaccination ?

Answer 9

• Injection of antigens
• From attenuated (dead or weakened) pathogens
• Stimulates the formation of memory cells
• A vaccine can lead to symptoms because some of the pathogens might be alive / active /viable;
  therefore, the pathogen could reproduce and release toxins, which can kill cells

Question 10

How do vaccines provide herd immunity ?

Answer 10

• Large proportion but not 100% of population vaccinated against a disease – herd immunity
• Makes it more difficult for the pathogen to spread through the population because…
• More people are immune so fewer people in the population carry the pathogen / are infected
• Fewer susceptible so less likely that a susceptible / non-vaccinated individual will come into
  contact with an infected person and pass on the disease

Question 11

What are the ethical issues associated with the use of vaccines ?

Answer 11

• Tested on animals before use on humans → animals have a central nervous system so feel
  pain (some animal based substances are also used to produce vaccines)
• Tested on humans → volunteers may put themselves at unnecessary risk of contracting the
  disease because they think they’re fully protected e.g. HIV vaccine so have unprotected sex
  → vaccine might not work
• Can have side effects
• Expensive – less money spent on research and treatments of other diseases

Question 12

What is antigen variability an explanation for ?

Answer 12

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

Question 13

Explain the effect of antigen variability on disease

Answer 13

• Change in antigen shape (due to a genetic mutation)
• Not recognised by B memory cell → no plasma cells / antibodies
• Not immune
• Must re-undergo primary immune response → slower / releases lower concentration of antibodies
• Disease symptoms felt

Question 14

Explain the effect of antigen variability on disease prevention (vaccines)

Answer 14

• Change in antigen shape (due to a genetic mutation)
• Existing antibodies with a specific shape unable to bind to changed antigens / form
  antigen-antibody complex
• Immune system i.e. memory cells won’t recognise different antigens (strain)

Question 15

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

Answer 15

• A successful vaccination programme:
• Produce suitable vaccine
• Effective – make memory cells
• No major side effects → side effects discourage individuals from being vaccinated
• Low cost / economically viable
• Easily produced / transported / stored / administered
• Provides herd immunity
• Evaluating a conclusion that’s been made from a set of data / study
• If there is a scatter graph, the relationship between two variables may be a positive /negative
  correlation, or no correlation
• But correlation between two variables doesn’t always mean there’s a causal
  relationship – correlation could be due to change or another variable / factor
• Repeatability (when an experiment is repeated using the same method and equipment and
  obtains the same results)
• Have there been other experiments / studies showing the same?
• Validity (suitability of the investigative procedure to answer the question being asked)
• Does the data answer the question set out to investigate?
• Example: research project on potential vaccines to protect people against HIV used
  monkeys and a virus called SIV (which only infects monkeys and causes a condition
  similar to AIDS in them) . Scientists have questioned the value of the research because
  there may be differences between human and money responses / immune systems,
  and a vaccine developed against SIV may not work against HIV / may be (significant)
  differences between SIV and HIV
• Potential bias?

Question 16

What are monoclonal antibodies ?

Answer 16

• Monoclonal antibody = antibody produced from a single group of genetically identical (clones)
  B cells / plasma cells
• Identical structure
• Bind to specific complimentary antigen
• Have a binding site / variable region with a specific tertiary structure / shape
• Only one complementary antigen will fit

Question 17

Why are monoclonal antibodies useful in medicine ?

Answer 17

• Only bind to specific target molecules / antigens because…
• Antibodies have a specific tertiary structure (binding site / variable region) that’s complementary
  to a specific antigen which can bind/fit to the antibody

Question 18

Monoclonal antibodies: targeting medication to specific cell types by attaching
a therapeutic drug to an antibody

Answer 18

Example: cancer cell
1. Monoclonal antibodies made to be complementary to antigens specific to cancer cells → cancer
cells are abnormal body cells with different antigens (tumour markers)
2. Anti-cancer drug attached to antibody
3. Antibody binds / attaches to cancer cells (forming antigen-antibody complex)
4. Delivers attached anti-cancer drug directly to specific cancer cells so drug accumulates → fewer
side effects e.g. fewer normal body cells killed

Question 19

Monoclonal antibodies: medical diagnosis

Answer 19

Example: pregnancy test
- Pregnant women have the hormone hCG in their urine
- Urine test strip has 3 parts with 3 different antibodies
- Application area, position 1: antibodies complementary to hCG (bound to a blue
coloured bead)
- Middle, position 2: antibodies complementary to hCG-antibody complex
- End, position 3: antibodies complementary to antibody without hCG attached
- 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 doesn’t bind to antibodies in application area so doesn’t
bind to antibodies at position 2 = no blue line
- Bind to antibodies at position 3 → blue line = control

Question 20

What is the ELISA test used for ?

Answer 20

To determine if a patient has
a) Antibodies to a certain antigen
b) Antigen to a certain antibody

Question 21

What are ethical issues associated with the use of monoclonal antibody ?

Answer 21

• Animals are involved in the production of monoclonal antibodies i.e. by producing cancer
  in mice who have a CNS so feel pain, and it is unfair to give them a disease
• Although effective treatment for cancer and 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 22

How does HIV work/replicated ?

Answer 22

1. HIV infects T helper cells (host cell)
   - HIV attachment protein (GP120) attaches to a receptor on
   the helper T-cell membrane
2. Virus lipid envelope fuses with cell surface membrane and capsid
   released into cell which uncoats, releasing RNA and reverse
   transcriptase into cytoplasm
3. Viral DNA is made from viral RNA
   - Reverse transcriptase produces a complementary viral DNA
   strand from viral RNA template
   - Double stranded DNA is made from this (DNA polymerase)
4. Viral DNA integrated into host cell’s DNA (by enzyme integrase)
5. This remains latent for a long time in host cell until activated
6. Host cell enzymes used to make viral proteins from viral DNA
   (within human DNA) → viral proteins assembled with viral RNA to
   make a new virus
7. New virus bud from cell (taking some of cell surface membrane as
   envelope)
8. Eventually kills helper T cells
9. Most host cells are infected and process repeat

Question 23

How does HIV cause AIDS ?

Answer 23

• Infects and kills helper T cells (host cell) as it multiplies rapidly
• T helper cells then can’t stimulate cytotoxic T cells, B cells and phagocytes →
  impaired immune response
• E.g. B plasma cells can’t secrete antibodies for agglutination and destruction
  of pathogens by phagocytosis
• Immune system deteriorates
• More susceptible to infections
• Diseases that wouldn’t cause serious problems in healthy immune system are deadly
  (opportunistic infections) e.g. pneumonia

Question 24

Why are antibiotics ineffective against viruses ?

Answer 24

• Antibiotics can’t enter human calls – but viruses exists in its host cell (they are acellular)
• Viruses don’t have own metabolic reactions e.g. ribosomes (use of the host cell’s) which
  antibiotics target
• If we did use them… act as a selection pressure + gene mutation = resistant strain of
  bacteria via natural selection → reducing effectiveness of antibiotics and waste money