Topic 2C - Cells And The Immune System

Question 1

What are antigens?

Answer 1

Proteins that can generate an immune response when detected by the body. They are on the surface of cells.

Question 2

Describe what a phagocyte is. Where are they?

Answer 2

A type of white blood cell that carries out phagocytosis (engulfment of pathogens). They’re found in the blood and in tissues and they’re the first to respond to an immune system trigger inside the body.

Question 3

How does a phagocyte work?

Answer 3

They recognise the foreign antigens on a pathogen. The cytoplasm/cell membrane engulfs the pathogen. The pathogen is now contained in a vesicle/phagosome (phagocytic vacuole) in the cytoplasm. A lysosome fuses with the phagosome, and the lysozymes break down/digest the pathogen. The phagocyte then presents the pathogen’s antigens (sticks the antigens on its surface to active other immune system cells).

Question 4

What is a T-cell?

Answer 4

A type of white blood cell. They have receptor proteins on its surface that bind to complementary antigens presented to it by phagocytes. This activates the T-cell.

Question 5

Name two types of T-cells and what they do.

Answer 5

Helper t-cells release chemical signals that activate and stimulate phagocytes, and b-cells and Cytotoxic t-cells that kill abnormal and foreign cells.

Question 6

What are B-cells?

Answer 6

A white blood cell covered with antibodies - proteins that bind to antigens to form an antigen-antibody complex.

Question 7

What’s unique about b-cells?

Answer 7

Each b-cell has a different shaped antibody on its membrane, so different ones bind to different shaped antigens.

Question 8

How do b-cells work?

Answer 8

When the antibody on the surface of a b-cell meets a complementary shaped antigen, it binds to it. This, together with the substances released from helper t-cells, activates the b-cell. This is clonal selection. The activated b-cell clones into plasma cells. Plasma cells produce monoclonal antibodies against the virus. Plasma cell produces memory cells.

Question 9

What are plasma cells? How do they work?

Answer 9

They’re clones of b-cells. They secrete antibodies specific to the antigen, called monoclonal antibodies, which bind to the antigens of the pathogen to form antigen-antibody complexes.

Question 10

Describe the structure and function of an antibody.

Answer 10

An antibody has two binding sites, so can bind to two pathogens at a time and clump them - agglutination. Phagocytes bind to the antibodies and phagocytise many pathogens at once. Antibodies are proteins. The specificity of an antibody depend on its variable regions, forming the antigen binding sides. Every antibody has a variable region with a unique tertiary structure. All antibodies have the same constant regions.

Question 11

What is the cellular immune response?

Answer 11

The t-cells and other immune system cells that they interact with, such as phagocytes and cytotoxic t-cells.

Question 12

What is the humoral immune response?

Answer 12

B-cells, clonal selection, and production of monoclonal antibodies.

Question 13

How do the cellular and humoral responses interact with each other?

Answer 13

T-cells help to activate b-cells. Antibodies coat pathogens, making it easier for phagocytes to engulf them.

Question 14

What is the primary immune response?

Answer 14

When an antigen enters the body for the first time.

Question 15

What happens in the primary immune response?

Answer 15

It is slow because there aren’t many B-cells that can make the antibody that binds to the antigen. Infected person will show symptoms. After being exposed to an antigen, t-cells and b-cells produce memory cells that remain in the body for a long time. Memory t-cells remember the specific antigen and will recognise it. Memory b-cells record the specific antibodies needed to bind to the antigen. The person becomes immune.

Question 16

What happens in the secondary immune response?

Answer 16

Quicker, stronger immune response. Clonal selection happens faster. Memory b-cells are activated and divide into plasma cells that produce the right antibody to the antigen. Memory t-cells are activated and divide into the correct type of t-cell to kill the cell carrying the antigen. Usually no symptoms shown.

Question 17

How do vaccines work?

Answer 17

Contain antigens that cause your body to produce memory cells against a particular pathogen, without the pathogen causing disease. It can also contain mRNA that codes for the antigens found on a pathogen.

Question 18

What is herd immunity?

Answer 18

Because vaccines reduce the occurrence of a disease, people who are not vaccinated are also less likely to catch the disease as there are fewer people to catch it from.

Question 19

What is the state of the antigens in a vaccine?

Answer 19

They can be free, or attached to a dead or attenuated (weakened) pathogen.

Question 20

What is the disadvantage of taking a vaccine orally?

Answer 20

Enzymes in the gut can break it down, or the molecules of the vaccine may be too large to be absorbed into the blood.

Question 21

What is antigenic variation?

Answer 21

Some pathogens change their surface antigens. This antigen variability is antigenic variation. Different antigens are formed due to changes in the genes of a pathogen.

Question 22

Why does the influenza vaccine change every year?

Answer 22

The antigens on the surface change regularly, forming new strains. Memory cells produced from vaccination with one strain of the flu will not recognise other strains with different antigens. The strains are immunologically distinct.

Question 23

What is active immunity?

Answer 23

Immune system makes its own antibodies after being stimulated by an antigen. Natural is when you become immune after catching a disease. Artificial is when you become immune after being given a vaccination.

Question 24

What is passive immunity?

Answer 24

Given antibodies made by a different organism. Natural is when a baby becomes immune due to the antibodies it receives from the mother, through the placenta and breast milk. Artificial is when you’re injected with antibodies from someone else.

Question 25

What are the differences between active and passive immunity?

Answer 25

Active requires exposure to antigen, but passive doesn’t. Active requires a while for protection to develop, but passive protection is immediate. Active produces memory cells but passive doesn’t. Protection is long term in active because the memory cells are activated, but protection in passive is short term since the antibodies are broken down. Active immunity is slower than passive immunity.

Question 26

What are monoclonal antibodies?

Answer 26

Antibodies produced from genetically identical b-cells (plasma cells) clones.

Question 27

How do you target cancer cells using antibodies?

Answer 27

Cancer cells have antigens called tumour markers. Monoclonal antibodies can be made that bind to the tumour markers. You can also attach anti-cancer drugs to the antibodies. The drug will only accumulate in the body where there are cancer cells. Side effects of an antibody-based drug are lower than other drugs because they accumulate near specific cells.

Question 28

How are monoclonal antibodies used in pregnancy testing?

Answer 28

Application area contains antibodies for hCG bound to a blue coloured bead. When urine is applied to the application area, any hCG will bind to the antibody on the beads, forming an antigen-antibody complex. The urine moved up the stick to the test strip, carrying any beads with it. The test strip contains antibodies to hCG that are immobilised. If there is hCG present, the test strip turns blue because the immobilised antibody binds to any hCG - concentrating the hCG - antibody complex with the blue beads attached. If no hCG, beads will pass though the test area without binding to anything.

Question 29

How does the ELISA test work?

Answer 29

An antibody is used which has an enzyme attached to it. This enzyme can react with a substrate to produce a coloured product, causing the solution in the reaction vessel to change colour. If there’s a colour change, it demonstrates that the antigen or antibody of interest is present. Intensity of the colour change can indication quantity of antigen/antibody.

Question 30

Describe the indirect ELISA test for HIV.

Answer 30

HIV antigens are bound to the bottom of a well in a well plate. A sample of the patient’s blood plasma is added to the well, and any HIV-specific antibodies will bind to the HIV antigen stick to the bottom of the well. The well is washed to remove any unbound antibodies. Then a secondary antibody with an enzyme attached to it is added to the well. This can bind to the HIV-specific antibody (primary antibody). The well is washed out. If there’s no HIV-specific antibody, the secondary antibodies are washed away. A solution is added to well containing a substrate, which reacts with the enzyme attached to the secondary antibody and produces a coloured product.

Question 31

What’s important when a new theory is presented?

Answer 31

Other scientists have to come up with more evidence to validate the theory. The study may be repeated to try and reproduce the same results, or other studies can be conducted to prove the same theory.

Question 32

What are ethical issues surrounding vaccines?

Answer 32

All vaccines are tested on animals before testing on humans. Animal based substances may be used to produce a vaccine. Volunteers who take vaccines may put themselves at unnecessary risk because they think they’re protected. Some people don’t want to take the vaccine due to the risk of side effects, but they’re still protected from herd immunity and some people think this is unfair. If there was an epidemic of a new disease, who would get the first vaccine?

Question 33

What is the ethical issue with monoclonal antibody therapy?

Answer 33

Animals are used to produce the cells from which the monoclonal antibodies are produced.

Question 34

What is HIV?

Answer 34

Human immunodeficiency virus. It affects the immune system and leads to AIDS.

Question 35

How does HIV work?

Answer 35

It infects and kills helper t-cells, which act as the host cells for the virus. This means the t-cells can’t activate phagocytes, cytotoxic t-cells and b-cells. AIDS occurs when helper t-cell numbers are critically low.

Question 36

Describe the structure of HIV.

Answer 36

A core containing RNA and proteins (like reverse transcriptase, needed for virus replication). Outer coating of protein called capsid (looks like a weird square around the RNA). Extra outer layer called envelope, made of membrane stolen from the cell membrane of a previous host cell. Attachment proteins helping it attach to the host helper t-cell.

Question 37

What doesn’t HIV have that normal cells have?

Answer 37

Enzymes, ribosomes.

Question 38

How does HIV replicate?

Answer 38

The attachment protein attaches to a receptor molecule on the cell membrane of the host helper t-cell. The capsid is released into the cell, where it uncoats and releases the RNA into the cell’s cytoplasm. Inside the cells reverse transcriptase is used to make a complementary strand of DNA from the viral RNA template. This creates double-stranded DNA which is inserted into the human DNA. Host cell enzymes are used to make viral proteins from the viral DNA found within the human DNA. The viral proteins are assembled into new viruses, which bud from the cell and infect other cells.

Question 39

What is the latency period?

Answer 39

When HIV drops to a lower level and the infected person won’t experience any symptoms.

Question 40

What are symptoms of AIDS?

Answer 40

Initial symptoms include minor infections of mucous membranes (e.g inside of the nose, ears and genitals), and recurring respiratory infections. As AIDS progresses, patients become susceptible to more serious infections.

Question 41

Why don’t antibiotics work against viruses?

Answer 41

Antibiotics kill bacteria by interfering with their metabolic reactions. They target the bacterial enzymes and ribosomes, which are different to human enzymes and ribosomes. Viruses don’t have these, so antibiotics can’t inhibit them because they don’t target human processes.

Question 42

How do most antiviral drugs work?

Answer 42

They are designed to target the few virus-specific enzymes. For example, HIV uses reverse transcriptase, whereas humans don’t, so drugs can be designed to inhibit it without affecting the host cell. These drugs are called reverse-transcriptase inhibitors.

Question 43

How does an ADC enter and kill a tumour cell? Similar to phagocytosis.

Answer 43

Cell membrane engulfs the antibody.
Lysosome fuse with vesicle contains ADC.
Lysozymes break down ADC to release the drug.

Question 44

Mice were injected with 2 mg kg^-1 of monoclonal antibody, which was in a solution of 500 mg dm^-3. Calculate the volume of antibody solution injected into a 23g mouse. Give your answer in dm^3.

Answer 44

We need to use the formula volume = mass/conc

First find mass of mouse in kg : 23/1000
We know there is 2mg per kg, so do (2 x 23/1000)/500 to get 9.2 x 10^-5.

Question 45

Suggest a reason there is no more data on a graph.

Answer 45

Species being measured died.

Question 46

Suggest and explain two further investigations that should be done (after an animal test) before a drug is tested on patients.

Answer 46

Test on healthy humans to check for side effects.
Investigate different concentrations to find suitable dosage.

Question 47

Name two types of cells other than pathogens that can stimulate an immune response.

Answer 47

Cells from other organisms/transplants.
Tumour cells.

Question 48

What is the role of the disulfide bridges in forming the quaternary structure of an antibody?

Answer 48

Joins the two polypeptides.

Question 49

How could a virus that could only infect one species of frog, be able to infect other species?

Answer 49

Mutation in viral RNA.
Altered tertiary structure of viral attachment protein.
Allows attachment proteins to bind to receptors of other species.

Question 50

How can determining the genome of the viruses allow scientists to develop a vaccine?

Answer 50

Analysing genome can show what proteins the viruses produced. Then identify the potential antigens to use in the vaccine.

Question 51

What is a monoclonal antibody?

Answer 51

Antibody produced by cloned b-cells.

Question 52

How does antivenom work? Why is it essential to use passive immunity rather than active immunity?

Answer 52

Antivenom antibodies bind to toxin antigen and cause its destruction. Active immunity is too slow.

Question 53

Why are a mixture of venoms from several snakes of the same species used in the production of antivenom?

Answer 53

May be different forms of antigens in one species. Different antibodies needed in the antivenom.

Question 54

Why do vets observe animals when producing antivenom?

Answer 54

So that the animal does not suffer as a result of blood collection.

Question 55

During vaccination, a small volume of venom is injected. Then a large volume of venom is injected. Use your knowledge of the humoral immune response to explain this.

Answer 55

B-cells specific to the venom reproduce by mitosis.
B-cells produce plasma cells and memory cells.
The second dose produces antibodies in the secondary immune system in higher concentration and quickly.

Question 56

Describe how HIV is replicated once inside helper T cells.

Answer 56

RNA converted into DNA using reverse transcriptase.
DNA inserted into nucleus
DNA transcribed into HIV mRNA
HIV mRNA translated into viral proteins for assembly into viral particles.

Question 57

What is a monoclonal antibody?

Answer 57

Antibodies with the same tertiary structure.
OR
antibody produced from identical plasma cell.

Question 58

The scientists hypothesised that memory B cells had formed in the mice 180 days after the 3rd injection. Suggest and explain a practical method to test this hypothesis.

Answer 58

Inject vaccine again
Immune response is quicker.

Question 59

Explain how the treatment with antivenom works and why it is essential to use passive immunity, rather than active immunity.

Answer 59

Antibodies bind to the antigen.
Active immunity would be too slow.