May/June 2026 Paper 23

Complete the passage by using the most appropriate scientific terms. Tetanus is unlike other diseases for which vaccines are available because it cannot be passed from an infected person to another person. Tetanus is therefore not considered to be a ___ disease. However, the bacterium C. tetani can be described as a ___ because it causes a disease when it infects an individual. C. tetani can enter the human body when a person has a wound that makes contact with substances that contain C. tetani, such as soil or dust.

C. tetani releases a toxin, which produces the symptoms of tetanus in an infected person. The tetanus toxin is a protein, with part of its structure functioning as an enzyme. Figure 4.1 shows the simplified structure of the tetanus toxin.

Describe the structure of the tetanus toxin shown in Figure 4.1. In your answer, you should link the descriptions to the different levels of protein structure shown in Figure 4.1.

The tetanus toxin enters nerve cells (neurones), where it causes the symptoms of the disease. Figure 4.2 shows the entry of the tetanus toxin into a neurone.

State the name of the process by which the toxin enters the neurone, as shown in Figure 4.2.

The symptoms of tetanus can develop in a person very quickly after infection by C. tetani. People can gain immunity to infection by C. tetani in a number of different ways. For each description, select the two types of immunity that apply.

Table 4.2 lists some events that occur during a primary immune response to infection by C. tetani. Each event is represented by a letter.

State the sequence of letters that shows a correct order of events that occur during an immune response.