Worked Solutions
Module 7: Infectious Disease — Worked Solutions (HSC Biology)
Created with Intu AI Reviewed by Intuition's expert tutors
Studying this? See our HSC Biology course →
Worked examples for HSC Biology Module 7: Infectious Disease. Each shows where the marks are awarded, the key idea, and a full model answer explained by your choice of tutor — Stella, Ella or Cassie.
How to use these
Attempt each question first, then check your answer against the model responses. Use the tutor tabs to read the solution in the style that suits you: Stella is direct and challenging, Ella is warm and explains the why, and Cassie is concise and analytical.
Infectious disease questions reward precise terminology. Name the cells and molecules involved, distinguish specific from non-specific responses, and link each step to how it protects the host.
Example 1 — The specific immune response
Question
Distinguish between the humoral and cell-mediated components of the specific (adaptive) immune response, identifying the cells involved and how each component eliminates a pathogen.
Solution
Both are part of the specific immune response and both depend on lymphocytes recognising a specific antigen, but they target pathogens differently.
Humoral response: driven by B lymphocytes. When activated, B cells differentiate into plasma cells that secrete antibodies. Antibodies circulate in body fluids and bind to antigens on free (extracellular) pathogens, neutralising them and marking them for destruction. This component targets pathogens in the blood and tissue fluid.
Cell-mediated response: driven by T lymphocytes. Cytotoxic (killer) T cells recognise and destroy the body's own infected cells — for example cells harbouring a virus — by triggering their death. This component targets intracellular pathogens.
So the humoral response uses antibodies against free pathogens, while the cell-mediated response uses T cells to kill infected host cells.
The specific immune response has two arms because pathogens hide in two different places — either floating freely in body fluids, or inside our own cells — and we need a way to deal with each.
The humoral response handles the free-floating pathogens. It's run by B lymphocytes. Once a B cell is activated by its matching antigen, it becomes a plasma cell, and plasma cells are antibody factories. The antibodies travel through the blood and tissue fluid, bind to the pathogen's antigens, and either neutralise the pathogen or flag it so other cells destroy it. Think of it as targeting the enemy out in the open.
The cell-mediated response handles pathogens that have already gotten inside our cells. It's run by T lymphocytes, especially cytotoxic T cells. These recognise the body's own infected cells and destroy them, removing the hiding place along with the pathogen. Think of it as dealing with the enemy inside the walls.
So the difference comes down to target and mechanism: B cells and antibodies for free pathogens (humoral), T cells killing infected cells for intracellular pathogens (cell-mediated).
Humoral response:
- Cells: B lymphocytes → plasma cells
- Mechanism: secrete antibodies into body fluids
- Target: free (extracellular) pathogens — neutralise/mark for destruction
Cell-mediated response:
- Cells: T lymphocytes (cytotoxic T cells)
- Mechanism: recognise and destroy infected host cells
- Target: intracellular pathogens (e.g. virus-infected cells)
Both are specific (antigen-driven); they differ in target location and mechanism.
Where the marks go
- 1 mark: Identifies B lymphocytes (plasma cells) as central to the humoral response
- 1 mark: States that the humoral response produces antibodies that act on free pathogens
- 1 mark: Identifies T lymphocytes (cytotoxic T cells) as central to the cell-mediated response
- 1 mark: States that the cell-mediated response destroys infected host cells / intracellular pathogens
- 1 mark: Clearly distinguishes the two components by cells and mechanism
Key idea
The humoral response uses B cells and antibodies against free pathogens; the cell-mediated response uses T cells to destroy infected host cells.
Example 2 — Vaccination and herd immunity
Question
Explain how vaccination produces immunity in an individual, and how widespread vaccination can protect even unvaccinated members of a population.
Solution
Individual immunity: a vaccine introduces antigens from a pathogen (weakened, killed or a fragment) without causing the disease. This triggers the specific immune response — B and T lymphocytes are activated and memory cells are formed. If the real pathogen later enters the body, the memory cells produce a faster, stronger secondary response, destroying the pathogen before symptoms develop.
Herd immunity: when a high proportion of a population is vaccinated, there are few susceptible hosts left for the pathogen to infect and spread between. This breaks the chain of transmission, so even unvaccinated individuals are unlikely to be exposed.
So vaccination protects the individual through immunological memory and protects the wider population by reducing pathogen transmission.
Let's split this into the individual and the population.
For the individual, the clever part of a vaccine is that it shows the immune system the pathogen's antigens without making you sick — using a weakened, killed, or fragment form. Your B and T lymphocytes respond as if it were a real infection, and crucially they form memory cells. These memory cells stick around. So if you later meet the actual pathogen, your immune system mounts a much faster and stronger secondary response, clearing the pathogen before you ever feel ill.
For the population, this is where herd immunity comes in. A pathogen spreads by jumping from host to host. If most people are vaccinated, the pathogen keeps running into immune individuals and can't find new hosts to infect. That breaks the chain of transmission. Because of this, even people who aren't vaccinated — like newborns or those who can't be vaccinated for medical reasons — are protected, simply because the disease can't circulate.
So the individual is protected by memory, and the community is protected by stopping spread.
Individual immunity:
- Vaccine introduces antigens (weakened/killed/fragment), no disease
- Activates specific immune response (B and T lymphocytes)
- Forms memory cells → faster, stronger secondary response on real exposure
Herd immunity:
- High vaccination rate → few susceptible hosts
- Breaks chain of transmission
- Unvaccinated individuals protected as pathogen can't circulate
Where the marks go
- 1 mark: States that a vaccine introduces antigens without causing disease
- 1 mark: Explains the formation of memory cells and a faster secondary response
- 1 mark: Explains that widespread vaccination reduces the number of susceptible hosts / breaks transmission
- 1 mark: Concludes that unvaccinated individuals are protected through herd immunity
Key idea
Vaccination builds individual immunity via memory cells, and high population coverage produces herd immunity by breaking pathogen transmission.