Chapyer 11.2

Question 1

Antibodies: Structure

Answer 1

• Antibodies are globular glycoproteins called immunoglobulins
• Antibodies have a quaternary structure (which is represented as Y-shaped), with two ‘heavy’ (long) polypeptide chains bonded by disulfide bonds to two ‘light’ (short) polypeptide chains
• Each polypeptide chain has a constant region and variable region
• The constant regions do not vary within a class (isotype) of antibodies but do vary between the classes. The constant region determines the mechanism used to destroy the antigens
• There are 5 classes of mammalian antibodies each with different roles

Question 2

A model of the generalised structure of an antibody molecule

Answer 2

Question 3

Function of antibodies

Answer 3

• Antibodies can combine with viruses and toxins of pathogens (e.g. bacteria) to block them from entering or damaging cells
• Antibodies can act as anti-toxins by binding to toxins produced by pathogens (e.g. the bacteria that cause diphtheria and tetanus) which neutralises them making them harmless
• Antibodies can attach to bacteria making them readily identifiable to phagocytes, this is called opsonisation. Once identified, the phagocyte has receptor proteins for the heavy polypeptide chains of the antibodies, which enables phagocytosis to occur
• Antibodies can attach to the flagella of bacteria making them less active, which makes it easier for phagocytes to do phagocytosis
• Antibodies act as agglutinins causing pathogens carrying antigen-antibody complexes to clump together (agglutination). This reduces the chance that the pathogens will spread through the body and makes it possible for phagocytes to engulf a number of pathogens at one time
• Antibodies (together with other molecules) can create holes in the cell walls of pathogens causing them to burst (lysis) when water is absorbed by osmosis

Question 4

Antibodies are produced by and what do they do (intro)

Answer 4

B-lymphocytes

Antibodies bind to specific antigens that trigger the specific immune response. Every antigen has one antibody

Antigens include pathogens and their toxins, pollen, blood cell surface molecules and the surface proteins found on transplanted tissues

Antibodies are divided into five major classes (isotypes), each with a different role

Question 5

The functions of antibodies vary according to which type of antigen they act on

Answer 5

Question 6

The amino acid sequence in the variable regions of the antibodies and the name of the variable region is called

Answer 6

the tips of the “Y”) are different for each antibody. The variable region is where the antibody attaches to the antigen to form an antigen-antibody complex

At the end of the variable region is a site called the antigen-binding site. Each antigen-binding site is generally composed of 110 to 130 amino acids and includes both the ends of the light and heavy chains

The antigen-binding sites vary greatly giving the antibody its specificity for binding to antigens. The sites are specific to the epitope (the part of the antigen that binds to the antibody)

A pathogen or virus may therefore present multiple antigens different antibodies need to be produced

Question 7

The ‘hinge’ region on an antibody

Question 8

Monoclonal antibodies

Answer 8

are artificially produced antibodies produced from a single B cell clone

Question 9

The hybridoma method is a method used to make

Answer 9

monoclonal antibodies (Mabs)

• The method enables large quantities of identical antibodies to be produced
• The hybridoma method solved the problem of having B cells that could divide by mitosis but not produce antibodies and plasma cells that could produce antibodies but not divide
• This method was established in the 1970s

Question 10

Monoclonal antibodies bind antigens

Answer 10

in the same way naturally produced antibodies do

Question 11

how are monoclonal antibodies produced

Answer 11

• They are produced by injecting mice with an antigen that stimulates the production of antibody-producing plasma cells
• Isolated plasma cells from the mice are fused with immortal tumour cells, which result in hybridoma cells
• These hybrid cells are grown in a selective growth medium and screened for the production of the desired antibody
• They are then cultured to produce large numbers of monoclonal antibodies
• Monoclonal antibodies have multiple applications to include diagnostics, treating disease, food safety testing and pregnancy testing

Question 12

The hybridoma method is used to produce monoclonal antibodies diagram

Answer 12

Question 13

Monoclonal antibodies can be used diagnostically for:

Answer 13

• Pregnancy tests
• Diagnosing HIV
• Detecting the presence of pathogens such as Streptococcus bacteria
• Distinguishing between Herpes I and Herpes II
• Blood typing before transfusions and tissue typing before transplants
• Detecting the presence of antibiotics in milk
• Detecting cancer cells

Question 14

Monoclonal antibodies can also be used to locate the position of blood clots for patients thought to have deep vein thrombosis. This occurs by:

Answer 14

• Injecting a mouse with human fibrin (the main protein found in blood clots)
• This activates the plasma cells to produce antibodies against fibrin
• These cells are collected from the mouse spleen
• The plasma cells are then fused with tumour cells forming hybridomas that produce antifibrin antibodies
• To detect where the antibodies are binding to fibrin molecules, a ra-dioactive chemical (producing gamma radiation) is attached to the antibodies making them radioactively labelled
• A gamma-ray camera is used to detect where these radioactively labelled antibodies have attached to a fibrin molecule, hence indicating where blood clots can be found

Question 15

general assumption of monoclonal

Answer 15

Generally monoclonal antibodies are used only once

Question 16

Another example of the diagnostic use of monoclonal antibodies – test for HIV

Answer 16

Question 17

Therapeutically monoclonal antibodies have multiple applications to include

Answer 17

• Treatment for the rabies virus, (which can be potentially fatal), by injecting purified antibodies
• The prevention of transplanted organ rejection, achieved by intervening with the T cells involved in the rejection process
• Autoimmune therapies for allergic asthma and rheumatoid arthritis; here monoclonal antibodies are able to bind and deactivate factors involved in the inflammatory response
• Treatment for diseases caused by the overproduction or inappropriate production of B-cells (eg. leukaemia, multiple sclerosis and myasthenia gravis); the antibody (rituximab) binds to cell surface receptor proteins on B-cells (not plasma cells) and causes the death of the cells
• Prevention of blood clotting following angioplasty procedures; here monoclonal antibodies bind to receptors on the platelet surface thereby inhibiting fibrinogen from binding and subsequent clotting from ensuing
• Targeted treatment of breast cancer; Herceptin (trastuzumab) is a monoclonal antibody used to treat breast cancer, it recognises receptor proteins on the surface of cancer cells and binds to them allowing the immune system to identify and destroy them
• Treatment of melanoma (a type of skin cancer); the antibody (ipilimumab) binds to a protein produced by T-cells (whose role is to reduce the immune response) which results in the immune system remain active against the cancer cells

Question 18

Using monoclonal antibodies as a treatment requires

Answer 18

• multiple administrations and this can cause problems
• Initially the monoclonal antibodies were produced by mice, rabbits or other laboratory animals (as these were easier to produce), however this triggered an immune response when they were introduced to humans

Question 19

Scientists have largely overcome trigger caused by monoclonal produced by animals by:

Answer 19

• Genetically modifying the antibody polypeptide chains so that the amino acid sequences are now human not mouse or rabbit sequences
• Altering the type and position of the sugar groups (antibodies are glycoproteins) attached to the heavy polypeptide chains to reflect those found on human antibodies

Question 20

Active immunity is

Answer 20

acquired when an antigen enters the body triggering a specific immune response (antibodies are produced)

• Active immunity is naturally acquired through exposure to microbes or artificially acquired through vaccinations
• The body produces memory cells, along with plasma cells, in both types of active immunity giving the person long-term immunity

Question 21

In active immunity, during the primary response to a pathogen (natural) or to a vaccination (passive)

Answer 21

the antibody concentration in the blood takes one to two weeks to increase. If the body is invaded by the same pathogen again or by the pathogen that the person was vaccinated against then, during the secondary response, the antibody concentration in the blood takes a much shorter period of time to increase and is higher than after the vaccination or first infection

Question 22

The primary and secondary response to the same antigen

Answer 22

Question 23

Passive immunity

Answer 23

is acquired without an immune response. Antibodies are not produced by the infected person

• As the person’s immune system has not been activated then there are no memory cells that can produce antibodies in a secondary response. If a person is reinfected they would need another infusion of antibodies
• Depending on the disease a person is infected with (eg. tetanus) they may not have time to actively acquire the immunity, that is, there is no time for active immunity. So passive immunity occurs either artificially or naturally

Question 24

Artificial passive immunity

Answer 24

occurs when people are given an injection / transfusion of the antibodies. In the case of tetanus this is an antitoxin. The antibodies were collected from people whose immune system had been triggered by a vaccination to produce tetanus antibodies

Question 25

Natural passive immunity occurs when:

Answer 25

• Foetuses receive antibodies across the placenta from their mothers
• Babies receive the initial breast milk from mothers (the colostrum) which delivers a certain isotype of antibody (IgA)

Question 26

Comparing Active & Passive Immunity Table

Answer 26

Question 27

what is a vaccine

Answer 27

is a suspension of antigens that are intentionally put into the body to induce artificial active immunity. A specific immune response where antibodies are released by plasma cells

Question 28

There are two main types of vaccines:

Answer 28

• Live attenuated
• Inactivated

Question 29

Vaccinations produce long-term

Answer 29

immunity as they cause memory cells to be created. The immune system remembers the antigen when reencountered and produces antibodies to it, in what is a faster, stronger secondary response

Question 30

Vaccines can be:

Answer 30

• Highly effective with one vaccination giving a lifetime’s protection (although less effective ones will require booster / subsequent injections)
• Generally harmless as they do not cause the disease they protect against because the pathogen is killed by the primary immune response

Question 31

Unfortunately there can be problems with vaccines:

Answer 31

• People can have a poor response (eg. they are malnourished and cannot produce the antibodies – proteins or their immune system may be defective)
• A live pathogen may be transmitted (e.g. through faeces) to others in the population (ideally enough number of people are vaccinated at the same time to give herd immunity)
• Antigenic variation
• Antigenic concealment

Question 32

Unfortunately there can be problems with vaccines: Antigenic variation

Answer 32

the variation (due to major changes) in the antigens of pathogens causes the vaccines to not trigger an immune response or diseases caused by eukaryotes (eg. malaria) have too many antigens on their cell surface membranes making it difficult to produce vaccines that would prompt the immune system quickly enough

Question 33

Unfortunately there can be problems with vaccines: Antigenic concealment

Answer 33

this occurs when the pathogen ‘hides’ from the immune system by living inside cells or when the pathogen coats their bodies in host proteins or by parasitising immune cells such as macrophages and T cells (eg. HIV) or by remaining in parts of the body that are difficult for vaccines to reach (eg. Vibrio cholerae – cholera, remains in the small intestine)

Question 34

Live attenuated vaccines

Answer 34

Live attenuated vaccines contain whole pathogens (e.g. bacteria and viruses) that have been ‘weakened’

• These weakened pathogens multiply slowly allowing for the body to recognise the antigens and trigger the primary immune response (plasma cells to produce antibodies)
• These vaccines tend to produce a stronger and longer-lasting immune response
• They can be unsuitable for people with weak immune systems as the pathogen may divide before sufficient antibodies can be produced
• An example of this type of vaccine is the MMR (Measles, Mumps and Rubella)

Question 35

Inactivated vaccines

Answer 35

• contain whole pathogens that have been killed (‘whole killed’) or small parts (‘subunit’) of the pathogens (eg. proteins or sugars or harmless forms of the toxins – toxoids)
• As inactivated vaccines do not contain living pathogens they cannot cause disease, even for those with weak immune systems

Question 36

do inactivated vaccines have a long term or short term efect

Answer 36

these vaccines do not trigger a strong or long-lasting immune response like the live attenuated vaccines. Repeated doses and / or booster doses are often required

• Some people may have allergic reactions or local reactions (eg. sore arm) to inactivated vaccines as adjuvants (eg. aluminium salts) may be conjugated (joined) to the subunit of the pathogen to strengthen and lengthen the immune response
• An example of a whole killed vaccine is polio vaccine
• An example of a toxoid subunit vaccine (where inactivated versions of the toxins produced by pathogens are used) is Diphtheria

Question 37

Eradicating disease presents a challenge

Answer 37

• On one hand some pathogens are simply complicated and present with disease processes that are not straightforward and so a successful vaccine has not been developed
• On the other hand, diseases that could be eradicated where a vaccine does exist, have not been eliminated because too few in the community have been vaccinated

Question 38

It has also been difficult to eradicate other infectious diseases due to:

Answer 38

• Unstable political situations in areas such as Africa, Latin America and parts of Asia, perhaps resulting in civil unrest or wars
• Lack of public health facilities (poor infrastructure, few trained personnel, limited financial resources)

Question 39

Tuberculosis

Answer 39

• The BCG vaccine for tuberculosis (TB) is not very effective (the immune response they trigger occurs too slowly) and it is very variable amongst populations; in those with latent (dormant) TB infections the vaccine does not prevent TB from developing
• Latent TB is challenging to treat and can become active TB at any time
• The BCG vaccine effectiveness decreases unless the person is exposed to TB

Question 40

Cholera

Answer 40

is caused by infection with a bacterium called Vibrio cholerae caused by eating or drinking contaminated water (in areas where there is poor hygiene and inadequate sanitation)

Question 41

Vaccination programmes have not eradicated cholera because:

Answer 41

• The vaccine only affords protection that is 50-60% effective, which decreases to less than 50% two years later
• There are many different strains of cholerae making it difficult to produce an effective vaccine
• If a cholera outbreak has started the 2-dose regimen (which are given a week apart and require a buffer solution) and the time required to reach protective efficacy is too great (about three weeks)
• The vaccine’s high cost

Question 42

Measles does actually fit the profile of a disease

Answer 42

in theory, should have already been eradicated:

• There is only one strain
• Humans are the only reservoirs
• Effective diagnostics exist (it can be detected easily)
• A successful vaccine exists, it has been implemented into the standard vaccine schedules given to children internationally and yet an estimated 160,000 children die of measles annually

Question 43

Unfortunately, although it is a preventable disease, it is still endemic in parts of the world and this is attributed to:

Answer 43

• Poor vaccination uptake – it is thought that 93 – 95% of the population need to be vaccinated for herd immunity to be achieved (that is, to prevent transmission in a population)
• Some children having a poor response to the vaccine and requiring several boosters
• Large cities with high birth rates and a shifting population making it hard to isolate and trace contacts of cases of measles
• Measles being highly communicable (infectious) with an R rate of 12-18 (so for every individual infected between 12 and 18 further individuals will become infected)
• Travellers reintroducing the measles virus to areas where it was previously deemed eliminated

Question 44

Malaria

Answer 44

• Vaccine attempts so far have had limited effectiveness as exposure to the parasite does not trigger a strong immune response (or it is even thought the parasite has the ability to interfere with the immune system) and long-term protection has not been possible. The vaccine being trialled currently also requires multiple doses (up to four), is applicable only to the Plasmodium parasite found in Africa and appears to be most effective on children that have reached their fifth month
• Also, as with all vaccine programs, the financial costs are high and this can impede progress

Question 45

Malaria has proven to be very difficult to eradicate, why?

Answer 45

this is because the Plasmodium parasite that causes it, has a complex life cycle and produces a multitude of genetic variants

Question 46

Smallpox

Answer 46

• Smallpox is a highly contagious disease caused by a virus that exists in two forms: Variola minor and Variola major, the later being the worst of the two, with a death rate of 12 to 30%
• Smallpox was transmitted by direct contact and caused red spots (which filled with pus) to cover the body. People who recovered were disfigured as a result of scabs that formed from these spots. It also affected the eyes resulting in permanent blindness for many who recovered

Question 47

The smallpox programme focused on

Answer 47

• Vaccination – the aim was to vaccinate more than 80% of populations at risk and if a case of smallpox was reported ring vaccination would occur (where everyone in the household with the reported case, the surrounding 30 households, relatives and anyone else who had contact would get vaccinated)
• Surveillance

Question 48

the success of the smallpox atributed to:

Answer 48

• the virus being stable – it did not mutate therefore its surface antigens did not change, therefore the same vaccine could be used worldwide which made it cheap to produce the vaccine
• The vaccine was a ‘live attenuated’ one, being produced from a harmless strain of a similar virus
• The vaccine could be transported without becoming unviable, as it could be freeze-dried and kept at high temperatures for up to 6 months, thus it was suitable for the tropics
• The symptoms made it easy to identify infected people (surveillance was possible)
• Humans being the only reservoirs of infection and there were no carriers making it easier to break the transmission pathway
• The consistency of the effort, vaccination, surveillance and containment of all outbreaks on a global scale