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. 

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. 

Myasthenia gravis is an autoimmune disease that causes muscle weakness by damaging neuromuscular junctions. 

What is meant by the term autoimmune disease? 

Suggest how the immune system causes damage to neuromuscular junctions.

Suggest why myasthenia gravis is a long-term condition.

Ciliated epithelial tissues in the airways of humans play an important role in defending the body against pathogens. 

Explain the function of ciliated epithelial tissues in the airways in defending the body against pathogens.

Phagocytes are cells of the immune system that form part of the body’s non-specific response to disease.

Explain why phagocytes are described as a secondary defence. 

Explain why phagocytes are described as being non-specific.

Another type of cell in the immune system produces molecules known as antibodies.

Describe how antibodies are involved in the processes of neutralisation and agglutination. 

The tomato plant, Solanum lycopersicum, is a vital crop in various parts of the world.

Tomatoes are susceptible to infection by a pathogen known as Fusarium oxysporum, which causes a disease called Fusarium wilt.

The most apparent sign of this disease is a yellowing and wilting of the leaves, often starting at the lower part of the plant.

Suggest a barrier that S. lycopersicum plants may have to make it difficult for F. oxysporum to enter the tomato cells. 

Give two ways in which a S. lycopersicum plant may respond to infection by F. oxysporum. 

The human body defends itself from pathogens using the process of phagocytosis.

Phagocytosis involves cytokines and opsonins.

Describe the role of cytokines and opsonins in phagocytosis.

Substances used to develop new medicines to treat diseases can be obtained from natural sources.

State two possible sources of these natural substances. 

Tuberculosis (TB) and malaria are both examples of communicable diseases. 

Name the types of pathogen that cause TB and malaria. 

Describe how the pathogen causing TB is transmitted from one individual to another.

The malarial pathogen is carried by the female Anopheles mosquito.

Explain how mosquitoes transmit the malarial pathogen to humans. 

Suggest why malaria is much more common in tropical areas of the world.