Questions on antibodies, vaccination and the four kinds of immunity

Question 1

During the 2014 Ebola outbreak in West Africa, an experimental drug that had protected monkeys, but had not been tested in humans, was given to a small number of patients. Evaluate the use of untested drugs during an epidemic.

Accepted Answer

B1 for: the disease kills a large proportion of those infected and there is no effective treatment, so a possible benefit may outweigh an unknown risk B1 for: a patient who understands the drug is unproven can consent to take the risk, and completing normal trials would take until after the epidemic B1 against: the drug has not passed phase I, so its safe dose and side effects are unknown and it could harm or kill patients B1 against: consent from a very ill, frightened patient with no alternative may not be freely given, and with a small supply someone has to decide fairly who receives it B1 against: without a comparison group no one learns whether the drug works, and resources may be drawn from proven measures such as isolation and rehydration B1 a judgement supported by the points, for example that use is acceptable with informed consent and with outcomes recorded, ideally within a randomised trial run during the epidemic

Question 2

Describe how a monoclonal antibody against a chosen antigen is produced, from the immunisation of a mouse to the harvesting of the antibody.

Accepted Answer

B1 the antigen is injected into a mouse, which mounts a specific immune response, and antibody-producing B lymphocytes are isolated from its spleen B1 those B lymphocytes are fused with myeloma cells, using polyethylene glycol or an electric pulse, and each cell formed from one of each is a hybridoma B1 the mixture is grown on a selective medium on which only the fused cells survive, so the unfused B lymphocytes and unfused myeloma cells are removed B1 the surviving hybridomas are grown separately and screened by testing their antibody against the antigen, because the mouse made B lymphocytes against many antigens and only some of the clones make the one wanted A1 a single positive hybridoma is cultured on a large scale, so every cell is a clone of one cell and every antibody molecule secreted is identical, and the antibody is harvested from the medium and purified

Question 3

An antibody has no enzymic activity of any kind. Explain how antibodies nevertheless help to clear a bacterial infection.

Accepted Answer

B1 an antibody has two binding sites and cannot lyse a cell, digest a wall or poison anything; all it does is bind an antigen to form an antigen-antibody complex B1 agglutination: because each antibody has two identical binding sites it can bind two bacteria at once, clumping them so they cannot spread and so one phagocyte can engulf many at a time B1 neutralisation: antibody bound over a toxin, or over the attachment proteins of a pathogen, covers the part that would otherwise fit a host receptor, so it cannot act or enter a cell B1 marking for phagocytosis: the constant regions project outwards and phagocytes carry receptors complementary to them, so the pathogen is engulfed and hydrolysed inside the phagocyte

Question 4

Compare active immunity with passive immunity, referring to how each arises, how quickly it acts and how long it lasts.

Accepted Answer

B1 in active immunity the person's own lymphocytes meet the antigen and respond, whereas in passive immunity ready-made antibody is introduced from outside B1 active immunity takes days or weeks to become established, whereas passive immunity works immediately because the antibody is already present B1 active immunity leaves memory cells behind, whereas passive immunity produces none B1 active immunity therefore lasts for years, whereas passive immunity fades within weeks to a few months as the introduced antibodies are broken down and nothing replaces them

Question 5

An ELISA test for HIV detects antibodies against the virus rather than the virus itself. Suggest why a person infected two weeks ago may still test negative, and suggest what follows for the screening of donated blood.

Accepted Answer

B1 after infection the specific response takes time: antigen must be presented, a B cell selected, and clonal expansion must run before plasma cells exist B1 antibody against HIV usually takes several weeks to reach a detectable concentration, a gap called the window period, so at two weeks it may still be below what the test can detect B1 the person is infected, and infectious, despite the negative result, which is a false negative B1 donated blood could therefore transmit HIV despite passing the screen, so an antibody test alone is not sufficient and a further test or a repeat test after an interval is needed

Question 6

A new vaccine is trialled during an epidemic, with one group given the vaccine and a control group given a placebo. Evaluate the use of a placebo control group in a trial of this kind.

Accepted Answer

B1 in favour: a placebo group provides a valid comparison, so any difference in infection rate between the groups can be attributed to the vaccine rather than to other factors B1 against: the control group is deliberately left without protection during an epidemic, so people who could have been protected may catch the disease B1 the alternative of comparing against an existing vaccine avoids that, but makes the trial larger, slower and more expensive, which delays the new vaccine for everyone B1 a judgement supported by the points made, for example that a placebo group is defensible only where no effective vaccine already exists and consent is genuinely informed

Question 7

Compare the response to the second injection with the response to the first, using values from the graph, and explain what the difference tells you about the cells involved.

Accepted Answer

B1 the second response peaks at 60 units against 12 units for the first, so it is five times higher B1 the second response reaches its peak about 7 days after the injection against about 14 days for the first, so it is faster B1 antibody from the second response is still well above the first response's peak when the graph ends, so it lasts longer A1 memory B cells made during the first response are already present and specific to the antigen, so they divide and differentiate into plasma cells at once rather than after clonal selection of a rare naive cell

Question 8

Describe the quaternary structure of an antibody molecule, and explain what its flexible hinge region allows it to do.

Accepted Answer

B1 an antibody has a quaternary structure of four polypeptide chains, two identical long heavy chains and two identical short light chains, held together by disulfide bridges into a Y shape B1 each arm carries one antigen-binding site, formed where a heavy chain and a light chain meet, and the two sites on one antibody are identical to each other B1 the hinge between the arms and the stem is flexible A1 this flexibility lets the two arms move to reach antigens that are not evenly spaced on a pathogen's surface, which helps the antibody bind two antigens at once

Question 9

Compare the way William Withering tested digitalis from foxglove with the way a new drug is tested today.

Accepted Answer

B1 Withering used dried leaf of variable strength, whereas a modern trial uses a pure compound at a measured dose B1 Withering tested safety and effectiveness together in sick patients, whereas a modern protocol tests safety in a few healthy volunteers first, in phase I, then effectiveness in phases II and III B1 Withering had no control group, whereas a modern trial compares the drug with a placebo or the existing treatment B1 Withering chose his patients and both he and they knew who was treated, whereas modern patients are allocated at random and the trial is double-blind

Question 10

A virus has a basic reproduction number R₀ of 12 in a fully susceptible population. Calculate the percentage of that population which must be immune before transmission is no longer sustained.

Accepted Answer

M1 uses the threshold proportion 1 − 1/R₀ M1 substitutes correctly: 1 − 1/12 = 1 − 0.0833 A1 0.917, that is 91.7 per cent (accept 92 per cent), of the population must be immune

Question 11

Explain why the influenza vaccine has to be reformulated every year while the measles vaccine does not.

Accepted Answer

B1 immunity is immunity to a shape: memory cells and antibodies are complementary to one antigen and to no other B1 the surface proteins of influenza, haemagglutinin and neuraminidase, accumulate mutations and whole genome segments can be exchanged, so its antigens change from year to year, which is antigenic variation B1 memory cells raised against last year's antigens bind this year's poorly or not at all, whereas measles has stable surface antigens, so the same memory cells still fit and childhood doses last for life

Question 12

Neither an antibody-producing B lymphocyte on its own nor a myeloma cell on its own can be used to manufacture a monoclonal antibody. Explain why each is unsuitable alone, and explain what fusing them achieves.

Accepted Answer

B1 a B lymphocyte from the immunised mouse's spleen makes the antibody that is wanted, but taken out of the body it divides only a few times in culture and then dies, so no useful quantity can be made from it B1 a myeloma cell divides indefinitely in culture, but it makes the wrong antibody or, in the lines chosen for fusion, none at all A1 the hybridoma formed by fusing them inherits both properties, so it secretes the wanted antibody and goes on dividing indefinitely, which is what allows a clone of identical cells to be grown from it

Question 13

Describe what a vaccine may contain, and describe what all vaccine types have in common.

Accepted Answer

B1 a vaccine may contain a weakened (attenuated) strain of the pathogen, an inactivated pathogen, a purified surface protein, a harmless toxin derivative, or the mRNA instructions for making one viral protein B1 what all of them have in common is that they supply antigen, or with mRNA the instructions for the body's own cells to make it A1 none of them contains a full dose of virulent pathogen, so a vaccine does not give a person a mild form of the actual disease

Question 14

A monoclonal antibody produced entirely in a mouse provokes an immune response against itself when given to a human patient. Explain how making the antibody chimeric or humanised reduces this problem.

Accepted Answer

B1 an antibody made entirely in a mouse is a mouse protein, so a human immune system recognises it as non-self and mounts a response against it B1 a chimeric antibody replaces the mouse constant region with a human constant region, keeping only the mouse variable regions that form the binding site A1 a humanised antibody goes further, keeping only the small part of the mouse sequence needed to form the binding site itself, with the rest human, so less of the molecule is recognised as foreign and the immune response against it is reduced

Question 15

Roughly one breast cancer in five over-produces a receptor protein called HER2 on the cell surface. Explain why a monoclonal antibody against HER2 affects cancer cells more than it affects healthy cells.

Accepted Answer

B1 the monoclonal antibody has binding sites complementary to HER2, so it binds specifically to cells carrying that receptor B1 cancer cells that over-produce HER2 carry far more of it on their surface than healthy cells do A1 so the antibody binds mostly to the cancer cells, blocking the growth signal HER2 carries and marking those cells for destruction, while healthy cells with little HER2 are largely unaffected

Question 16

The calculated herd immunity threshold for measles is 93 to 94%, yet public health services aim for 95% vaccine coverage. Explain why the coverage target is set higher than the calculated threshold.

Accepted Answer

B1 the calculated threshold assumes a population where everyone mixes evenly with everyone else and every vaccinated person becomes and stays fully immune B1 real populations are not like that: unvaccinated people cluster in particular schools or communities, and a small fraction of those vaccinated never respond properly to the vaccine A1 the 95% coverage target allows for these departures from the idealised assumptions, as well as for immunity that wanes over time

Question 17

Describe the three phases of clinical testing of a new drug.

Accepted Answer

B1 phase I: a small number of healthy volunteers, to test safety, the dose tolerated and side effects B1 phase II: a few hundred patients with the condition, to test whether the drug works and at what dose B1 phase III: thousands of patients in a randomised, usually double-blind trial, comparing the drug with a placebo or the existing treatment

Question 18

A baby receives antibodies in its mother's breast milk. A child of five is given the MMR vaccine. Identify the type of immunity produced in each case, naming both axes of the classification.

Accepted Answer

B1 the breast milk gives natural passive immunity B1 the MMR vaccine gives artificial active immunity

Question 19

In a phase III trial, neither the patients nor the doctors assessing them know who received the drug and who received the placebo. Explain why the trial is designed in this way.

Accepted Answer

B1 patients who believe they are being treated may improve anyway, the placebo effect, so patients not knowing means this is the same in both groups B1 doctors not knowing prevents their expectations affecting how they treat, examine or record patients, so bias in the results is avoided