QCE Biology · Unit 2
QCE Biology Unit 2 Topic 2: Infectious Disease and Immunity — Flashcards & Quiz
QCE Biology Unit 2 Topic 2 Infectious disease and epidemiology examines how infectious disease spreads through a population and how organisms resist it. These 20 free flashcards and 20 true/false quiz questions cover the major groups of pathogen (prions, viruses, bacteria, fungi, protists and parasites), how adherence factors, invasion factors, capsules and toxins cause disease, modes of transmission, the first line of defence formed by barriers, the innate second line including inflammation, phagocytosis, the complement system and natural killer cells, the adaptive third line involving B and T lymphocytes, clonal selection and immunological memory, primary and secondary responses, active and passive immunity, vaccination and herd immunity, innate defences in plants, and how epidemiological data and control measures are used to limit the spread of disease. Aligned to the QCAA Biology 2025 General senior syllabus.
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Key Terms
- Pathogen
- An organism or agent that causes disease in a host, including bacteria, viruses, fungi, protists and prions. The group determines both the mechanism of disease and the treatments that can work against it.
- Antigen
- A molecule, typically a protein or polysaccharide on a pathogen surface, that the immune system recognises as foreign and that triggers a specific adaptive response against it.
- Antibody
- A protein secreted by plasma cells derived from activated B lymphocytes, which binds to one specific antigen and acts by neutralising, agglutinating or marking the pathogen for destruction rather than killing it directly.
- Clonal selection
- The process by which an antigen activates only those lymphocytes whose pre-existing receptors match it, causing them to proliferate into a clone of effector cells and memory cells specific to that antigen.
- Memory cell
- A long-lived lymphocyte produced during an adaptive response that persists after the infection is cleared, allowing a much faster and larger secondary response on re-exposure to the same antigen.
- Herd immunity
- The indirect protection of susceptible individuals that occurs when a sufficiently large proportion of a population is immune, so chains of transmission break down and the pathogen cannot spread effectively.
- Complement system
- A group of blood proteins of the innate immune response that, once activated, coat pathogens for phagocytosis, attract phagocytes, enhance inflammation and can form pores that rupture pathogen membranes.
Sample Flashcards
Q1: What is a pathogen, and what distinguishes the major groups?
A pathogen is an organism or agent that causes disease in a host. Bacteria are single-celled prokaryotes that can reproduce independently and are often treatable with antibiotics. Viruses are not cellular and must use a host cell to replicate, so antibiotics do not affect them. Fungi and protists are eukaryotes causing infections such as thrush and malaria, while prions are misfolded proteins with no nucleic acid that induce misfolding in normal proteins. Parasites such as tapeworms, ticks and lice live on or in a host and draw nutrients from it.
Q2: List the main modes of pathogen transmission.
Direct contact transmits pathogens between individuals by touch or bodily fluids. Droplet and airborne routes carry pathogens in exhaled droplets or suspended particles. Vehicle transmission occurs through contaminated food, water or objects. Vector transmission uses another organism, commonly an insect, to carry the pathogen between hosts. Vertical transmission passes a pathogen from parent to offspring before or during birth.
Q3: What makes up the first line of defence?
The first line is a set of non-specific barriers that prevent pathogens entering the body at all. It includes the physical barrier of intact skin, mucus that traps particles and cilia that sweep them away, and chemical defences such as stomach acid, lysozyme in tears and saliva, and sebum on the skin. Beneficial resident microorganisms also compete with incoming pathogens for space and nutrients.
Q4: What characterises the second line of defence?
The second line is the innate internal response, which acts against any pathogen that breaches the barriers without distinguishing between species. It includes inflammation, phagocytosis by cells such as macrophages and neutrophils, fever, the complement system of blood proteins, and natural killer cells that destroy infected body cells. It responds within minutes to hours but generates no lasting memory.
Q5: Describe the inflammatory response and its purpose.
Damaged cells and mast cells release chemical signals including histamine, which cause nearby arterioles to dilate and capillaries to become more permeable. Increased blood flow produces redness and heat, while fluid leaving the capillaries causes swelling and, with the chemicals released, pain. The purpose is to deliver phagocytes and defensive proteins to the site quickly and to restrict the spread of the pathogen.
Q6: Outline the steps of phagocytosis.
The phagocyte is attracted to the site by chemical signals and recognises the pathogen as foreign. It engulfs the pathogen by extending its membrane around it, enclosing it in a vesicle called a phagosome. A lysosome fuses with the phagosome and releases digestive enzymes that break the pathogen down, and the debris is expelled. Some phagocytes then display fragments of the pathogen on their surface, linking the innate response to the adaptive one.
Q7: What is an antigen, and how does it differ from an antibody?
An antigen is a molecule, usually a protein or polysaccharide on the surface of a pathogen or cell, that the immune system can recognise as foreign and that triggers a specific response. An antibody is a protein produced by plasma cells derived from B lymphocytes, which binds to a specific antigen. Antigens provoke the response and antibodies are part of the response produced.
Q8: Distinguish the roles of B lymphocytes and T lymphocytes.
B lymphocytes are responsible for the antibody-mediated response. When activated they multiply and differentiate into plasma cells that secrete large quantities of specific antibody, and into memory B cells. T lymphocytes are responsible for the cell-mediated response: helper T cells release signals that coordinate and amplify the whole adaptive response, while cytotoxic T cells destroy the body's own cells that have become infected.
Sample Quiz Questions
Q1: Antibiotics are effective against viral infections.
Answer: FALSE
Viruses are not cellular and lack the structures antibiotics target, so antibiotics have no effect on them. They act against bacteria.
Q2: Malaria is transmitted by a vector.
Answer: TRUE
Correct. It is carried between hosts by mosquitoes, which is why control focuses on removing breeding sites and preventing bites.
Q3: Stomach acid and lysozyme in tears are part of the first line of defence.
Answer: TRUE
Correct. The first line includes chemical as well as physical barriers, all acting to prevent pathogens entering the body.
Q4: The second line of defence targets a specific pathogen and produces memory cells.
Answer: FALSE
The second line is innate and non-specific, responding the same way to any pathogen and generating no memory. Specificity and memory belong to the third line.
Q5: Swelling during inflammation is caused by fluid leaving capillaries that have become more permeable.
Answer: TRUE
Correct. Histamine increases capillary permeability, so fluid moves into the tissue and produces swelling.
Why It Matters
Infectious disease and epidemiology is where cell biology, population biology and public health meet, so this topic connects mechanisms you can see under a microscope to decisions made about whole communities. The same reasoning that explains why a secondary response is faster also explains why vaccination programs work, why herd immunity protects the vulnerable and how contact tracing and quarantine slow an outbreak, which makes it unusually rich territory for explaining and analysing data.
Key Concepts
Pathogens and Transmission
Pathogens fall into groups (prions, viruses, bacteria, fungi, protists and parasites) with different structures and vulnerabilities, and factors such as adherence, invasion, capsules and toxins determine how they cause disease. Transmission routes include direct contact, body fluids, contaminated food and water, and vectors, and effective control measures are those that interrupt the specific route in question.
The Three Lines of Defence
The first line consists of non-specific barriers preventing entry. The second is the innate internal response of inflammation, phagocytosis, the complement system and natural killer cells, which acts quickly against anything but generates no memory. The third is the adaptive response, which is specific to a particular antigen and produces lasting memory. Plants rely on innate physical and chemical defences only.
Adaptive Immunity and Clonal Selection
Lymphocytes each carry receptors of a single specificity generated before exposure. An antigen selects the matching cells, which proliferate into effector cells that fight the current infection and memory cells that persist. B cells give antibody-mediated immunity while T cells coordinate the response and destroy infected host cells.
Vaccination, Herd Immunity and Disease Control
Vaccination presents antigen safely so a primary response and memory cells develop without illness. When enough of a population is immune, transmission chains break and herd immunity protects those who cannot be vaccinated. Hygiene, contact tracing and quarantine further limit spread, and outbreak data can be analysed to find the source and judge which control strategy works.
Common Mistakes to Avoid
- Swapping antigen and antibody. The antigen is the foreign molecule that triggers the response, and the antibody is the protein produced against it.
- Calling the complement system or natural killer cells part of the adaptive response. Both are innate, non-specific defences that act against many pathogens and produce no memory.
- Describing a vaccine as supplying antibodies. A vaccine supplies antigen and stimulates the recipient's own response, producing memory cells.
- Treating the second line of defence as specific. It is innate and non-specific, responding identically to any pathogen and producing no memory.
- Saying antibodies kill pathogens directly. They neutralise, agglutinate and mark pathogens so that other components such as phagocytes and complement destroy them.
Study Tips
- Keep the three lines of defence in a single table with columns for specific or non-specific, speed, examples and whether memory is produced, so the boundaries between them are clear.
- Practise saying antigen generates and antibody attacks until the two words cannot be confused under exam pressure.
- Draw the primary and secondary response graph from memory, checking that the secondary curve is earlier, steeper and higher.
- Whenever you name a control measure, name the transmission route it interrupts in the same sentence.
- For each pathogen group (prions, viruses, bacteria, fungi, protists, parasites), learn one key feature and one example disease.
- Use vaccination to rehearse the whole topic: it touches antigens, clonal selection, memory cells, secondary responses and herd immunity in one example.
Related Topics
Exam Prep & Study Notes
Frequently Asked Questions
What does QCE Biology Unit 2 Topic 2 cover?
It covers infectious disease and epidemiology: infectious and non-infectious disease, the key features of prions, viruses, bacteria, fungi, protists and parasites, how adherence factors, invasion factors, capsules and toxins affect pathogenesis, recognition of self and non-self, the three lines of defence (including inflammation, the complement system and natural killer cells), the humoral and cell-mediated adaptive responses and memory cells, active and passive immunity, innate defences in plants, modes of transmission, factors affecting the spread of disease, and control measures such as hygiene, contact tracing and quarantine.
What is the difference between an antigen and an antibody?
An antigen is a molecule, usually on the surface of a pathogen, that the immune system recognises as foreign and that triggers a specific response. An antibody is a protein produced by plasma cells derived from B lymphocytes, which binds to one specific antigen. In short, antigens provoke the response while antibodies form part of the response produced against them.
Why do antibiotics not work against viruses?
Antibiotics act on structures and processes found in bacterial cells, such as the cell wall or bacterial protein synthesis. Viruses are not cellular and replicate using the machinery of the host cell, so those targets are absent and the drugs have no effect. Unnecessary antibiotic use also increases selection pressure for resistant bacteria.
Last updated: 9 October 2026 · 20 sample flashcards · 20 sample quiz questions · Content aligned to the QCAA Syllabus