The Immune System (CIE A Level Biology)

Lymphocytes and phagocytes are two groups of white blood cells involved in the immune response.

Contrast lymphocytes and phagocytes.

A group of scientists investigated the immunosuppressant action of a drug called cyclophosphamide. They used two groups of mice, one of which was a control group and the other of which was the treatment group. The treatment group was given a dose of 60 mg kg^-1 of cyclophosphamide. The scientists then carried out a cell count of two groups of cells. Their results are shown in Fig. 1.

Fig. 1

Calculate the percentage decrease in the number of lymphocytes between the control group of mice and the group treated with cyclophosphamide shown in Fig. 1. 

State the possible conclusions that can be drawn from Fig. 1.

Explain a possible medical application of a drug that is effective at suppressing the immune system.

Fig. 1 shows a series of T-lymphocytes, also known as T cells.

Fig. 1

Explain one feature of T-lymphocytes shown in Fig. 1 that allows them to carry out their role.

Fig. 2 shows changes in numbers of T-lymphocytes and pathogens during an infection.

Fig. 2

Explain the following in Fig. 2:

The change in the number of T-lymphocytes during the early days of the infection.

[2]

(ii)

The change in the number of pathogens that follows the change explained in part (i).

[2]

Explain how the secondary immune response to the pathogen would be different to the response shown in Fig. 2.

Fig. 1 shows B-lymphocytes during several stages of the immune response.

Fig. 1

For the stage labelled Y in Fig. 1:

(i)

Name the stage.

[1]

(ii)

Describe the events that take place during the stage.

[2]

Prior to stage Y in Fig. 1 the B-lymphocytes are shown to be activated. This is sometimes referred to as being stimulated.

State how B-lymphocytes are activated.

Fig. 2 shows the difference between an inactived and an activated, or stimulated, B-lymphocyte.

Fig. 2

Explain how the structure of the activated B cell enables it to carry out its function.

Suggest how each of the following diseases or conditions reduce the function of the immune system:

The parasite that causes malaria spends part of its life cycle inside red blood cells.

[1]

(ii)

The bacterium that causes tuberculosis inhibits the formation of lysosomes.

[1]

(iii)

Staphylococcus aureus bacteria release proteins that bind to antibodies.

[1]

(iv)

Some forms of severe combined immunodeficiency (SCID) are caused by a mutation in the genes that code for T-lymphocyte receptors.

[1]

The main cause of tuberculosis (TB) in humans is the bacterium Mycobacterium tuberculosis. Most cases of the disease involve the lungs. The bacterium can enter cells and remain inactive in a dormant state. However, the bacterium can become active to produce symptoms of the disease.

In a person with active TB, the pathogen can be present in airborne droplets that are exhaled. Generally, a healthy person who inhales these droplets has effective defence mechanisms in the gas exchange system to prevent infection.

One example of a defence mechanism against pathogens in the gas exchange system involves the action of macrophages.

(i)

Describe the mode of action of a macrophage against a bacterial cell.

[3]

(ii)

A healthy person has other defence mechanisms in the gas exchange system to prevent bacteria entering cells.

Describe these defence mechanisms and explain how bacteria in inhaled air are prevented from entering cells of the gas exchange system.

In people with a weakened immune system, M. tuberculosis can infect other organs and tissues, such as the kidneys and joints.

Suggest how the bacteria may spread from the lungs to other organs and tissues.

State the mode of transmission of this pathogen to humans.

Rifampicin is one antibiotic that can be used to kill mycobacterial cells. Although rifampicin is very effective at killing mycobacterial cells, it is often the first antibiotic to which resistance develops.

Rifampicin binds to a section of RNA polymerase that has attached to the DNA template strand.

Explain how binding to RNA polymerase allows rifampicin to kill mycobacterial cells.

The standard treatment for TB continues for six months. Initially, four antibiotics are prescribed. This is then reduced to two of the four antibiotics, rifampicin and isoniazid, if the person responds to treatment.

A person with multidrug-resistant TB (MDR-TB) does not respond to treatment with rifampicin and isoniazid. The treatment for MDR-TB involves other antibiotics and can last for up to 30 months.

Table 1 shows the number of reported cases of TB and MDR-TB in the South-East Asia region between 2005 and 2014, as published by the World Health Organization (WHO).

Table 1

Table 1 shows that between 2005 and 2014 there is an overall increase in the total number of reported cases for both TB and MDR-TB.

Describe the differences between the trend in the total number of reported cases for TB and the trend in the total number of reported cases of MDR-TB, as shown in Table 1.

Many social and economic factors need to be considered in the prevention and control of MDR-TB.

Outline three of these factors.

Fig. 1 shows a cell of the immune system. N denotes a region of the nucleus, A denotes a lysosome, and G denotes a glycogen granule.

Fig. 1

Identify the type of cell shown in Fig. 1.

[1]

(ii)

Explain how the cell shown in Fig. 1 is adapted to its function.

[2]

During chemotherapy treatment for cancer the cell count for the cell type shown in Fig. 1 can drop significantly.

Explain why this can be dangerous for patients.

A type of drug known as a colony stimulating factor (CSF) can be used to minimise the risk of low cell counts of the cell type shown in Fig. 1 for chemotherapy patients. 

A study was carried out into the effectiveness of a CSF drug. 225 patients took part in the trial, either receiving CSF or placebo treatment after undergoing chemotherapy. The cell count of the cell type shown in Fig. 1 was measured each day throughout the study.

Note that a placebo is a treatment that looks identical to the CSF treatment but that contains no active drug.

The results of the study are shown in Fig. 2.

Fig. 2

Suggest the purpose of a placebo in a drug trial of this kind.

A student saw Fig. 2 and concluded that the CSF drug being tested was effective in reducing the risks of chemotherapy to patients.

Evaluate the student's conclusion.

The microscope image in Fig. 1 shows a virus known as HIV. HIV can cause AIDS if left untreated.

CDC/Dr. Edwin P. Ewing, Jr., Public domain, via Wikimedia Commons

Fig. 1

Identify the structures labelled X and Y in Fig. 1.

Table 1 below contains information obtained from the analysis of blood samples. Group A consists of healthy individuals and group B has been diagnosed with HIV/AIDS, having not received treatment.

Table 1

Suggest and explain the symptoms that might be experienced by individuals in group B.

While treatments known as antiretroviral therapy (ART) are available to prevent the replication of HIV particles in the blood, scientists have been unable to fully eradicate HIV in patients using ART alone. This is because HIV can become dormant inside CD4⁺ T cells.

Research into a drug known as an anti-PD-1 checkpoint inhibitor suggests that this drug may have a role to play in the eradication of HIV from an infected individual.

Fig. 2 below shows the mechanism of action of anti-PD-1 checkpoint inhibitors.

Fig. 2

Suggest why the immune system is unable to eradicate HIV from the body prior to treatment with anti-PD-1 checkpoint inhibitors even when CD4⁺ T cell counts are high.

[2]

Suggest how treatment with anti-PD-1 checkpoint inhibitors may result in HIV eradication.

[2]

In another form of HIV treatment, patients may receive a stem cell transplant containing stem cells that produce daughter cells that are resistant to HIV. Note that this is not a commonly offered treatment due to the potential risks of stem cell transplants.

Suggest how a cell might be resistant to infection by HIV.