11.1 The Immune System

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.

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.