The diagram below shows the structure of an antibody. 

Using the diagram above, describe the evidence that this antibody has a quaternary structure.

Describe the role of the disulfide bridge labelled in the diagram above. 

Explain why antibodies will only bind with specific antigens. 

One type of cell involved in the immune response is known as a phagocyte. 

Describe how phagocytes destroy pathogens.

The immune response involves the identification of antigens on invading pathogens. 

What is an antigen?

Other than pathogens, give two types of cell that may stimulate an immune response.

During the immune response, phagocytes present a pathogen’s antigens on their cell surface. 

Describe how presentation of antigens leads to the release of antibodies against these antigens. 

Explain how the release of antibodies stimulates the process of phagocytosis. 

HIV is a disease caused by a virus.

The human immunodeficiency virus has a protein called gp120 on its surface, which binds to a specific receptor protein, CD4, on the surface membranes of human cells, particularly certain types of white blood cell. When it binds to this receptor protein, the virus can enter the cell.

Some individuals have a mutation in another receptor, CCR5, which HIV uses to enter cells. These people can become infected with HIV but are less likely to progress to full-blown AIDS.

The development of AIDS affects the number of antibodies produced by the immune system.  

HIV has a high mutation rate, which helps it evade the body's immune response. This makes it difficult to develop a vaccine.

Suggest why individuals with a mutation in the CCR5 receptor can become infected with HIV, but are less likely to develop AIDS. 

Explain why a high mutation rate makes it difficult to develop a vaccine. 

Explain how HIV affects the production of antibodies when AIDS develops. 

Explain how the destruction of T cells by HIV eventually leads to death of the affected individual.

Antibodies are used in the immune response to defend the body against pathogens. 

What is an antibody? 

Explain how vaccines can be used to protect people against disease.

Vaccination triggers individuals to develop active immunity to a disease. 

Explain the difference between active and passive immunity. 

When a person is stung by a venomous jellyfish, the jellyfish releases a potent venom into the person's skin.

Antivenom is used as a treatment for such stings.

Antivenom contains antibodies that neutralize the jellyfish venom, acting as an example of passive immunity.

Explain how the treatment with antivenom works and why it's crucial to use passive immunity, rather than active immunity, in this scenario. 

Antivenom is produced by injecting small amounts of venom from jellyfish into animals such as donkeys, then extracting antibodies from the donkey’s blood. 

Explain why it is beneficial to inject the donkey with jellyfish venom on more than one occasion.

Suggest a possible issue with injecting people with antivenom produced by donkeys. 

A mixture of venoms from several jellyfish of the same species are injected into animals when preparing antivenom solutions. 

Suggest why several jellyfish are used rather than just one. 

Norovirus is an infection caused by a virus that causes vomiting and diarrhoea.

Researchers wanted to investigate the effectiveness of a new vaccine against norovirus.

A doctor vaccinated a group of patients against norovirus. She gave each patient an initial dose and then a second dose three months later.

A scientist measured the concentration of antibodies in three samples of blood from each of the patients:

•  Sample 1 was taken 1 week before the first vaccine.

•  Sample 2 was taken 1 week after the first vaccine.

•  Sample 3 was taken 1 week after the second vaccine.

The results are shown in the graph.

What is a vaccine? 

Calculate the percentage increase in the average antibody concentration between samples 2 and 3. 

Suggest one factor the scientist should have considered when selecting patients for this trial.

Explain why blood samples 1, 2, and 3 show different antibody concentrations. 

Most cases of cervical cancer are caused by infection with human papillomavirus (HPV).

This virus can be spread by sexual contact. 

Scientists investigated the relationship between cervical cancer and infection with certain types of HPV.

The graph shows the frequency of five different types of HPV in women who had cervical cancer.

A vaccine made from HPV types 16 and 18 is offered to girls aged 12 to 13.

Suggest why these types are included in the vaccine, whereas others are not.

Explain why this vaccine may not protect individuals from HPV types 6, 11, and 31. 

More recently, the vaccine has also been offered to young men.

Explain why. 

Smear tests are offered to women in which abnormal cells in the cervix can be identified and removed before cervical cancer develops. 

Explain why smear tests continue to be offered to vaccinated women. 

The diagram below shows the structure of a human immunodeficiency virus (HIV). 

Name parts labelled A, B, and C in the diagram above. 

Using the diagram above, describe the structure of HIV. 

Viruses can only replicate once inside host cells.

Describe how HIV replicates inside helper T cells. 

A vaccine has not yet been developed to protect people against HIV.

One of the reasons for this is because HIV rapidly enters host cells in the body. 

Explain why the rapid entry of HIV into body cells makes it difficult to develop a vaccine.

Monoclonal antibodies can be used in both the diagnosis of certain diseases such as cancer. 

The protein CD20 is found on the surface of certain types of lymphoma cells.

A scientist used an anti-CD20 monoclonal antibody to identify lymphoma cells in a sample of lymph node tissue observed using an optical microscope. 

Name the part of the antibody molecule that will bind to the CD20 protein on lymphoma cells. 

Suggest how the monoclonal antibody helped the scientist to identify lymphoma cells in the sample of lymph node tissue.

Antibodies are often used in diagnosis using the ELISA test.

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

After diagnosis, monoclonal antibodies can be used as treatment for certain diseases.

Give one example of how monoclonal antibodies can be used as treatment.

Cholera and measles are both types of infectious diseases.

Cholera is an infection of the small intestine by Vibrio cholerae bacteria.

Measles is an airborne infection caused by a virus. 

Some diseases can be treated with antibiotics.

Give two factors, other than cost, that need to be considered when choosing an antibiotic to treat a disease. 

Antibiotics can be used to treat patients diagnosed with cholera.

Explain why antibiotics are not effective against measles. 

Penicillin was an antibiotic originally used to treat meningitis.

However, strains of the bacteria that cause meningitis have become resistant to penicillin.

Describe how a population of bacteria can become resistant to penicillin. 

Some strains of bacteria that cause meningitis are resistant to multiple antibiotics.

Discuss the consequences of this resistance for health authorities.