Fever feels terrible. Inflammation hurts. Pus is unpleasant. But every one of these responses is your body doing exactly what it should — a coordinated, biochemical assault on invading pathogens, running automatically the moment a barrier is breached.
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
When you get a cut that becomes infected, the area around it turns red, swells, feels warm, and hurts. This is inflammation.
Before reading: predict what is actually happening at the cellular level to cause each of these four signs — redness, swelling, warmth, and pain. Write your prediction for each one.
Come back to this at the end of the lesson.
Wrong: Homeostasis means the body stays exactly the same all the time.
Right: Homeostasis involves dynamic equilibrium — constant small adjustments around a set point.
Core Content
Before any immune response can occur, pathogens must first get past the body's physical barriers — structural features that prevent entry in the first place. These are the body's equivalent of the castle walls from L08.
| Barrier | Location | How It Prevents Infection |
|---|---|---|
| Skin (epidermis) | External body surface | Tough, keratinised, multilayered — physically blocks most pathogens; dead outer cells constantly shed, removing surface microbes; slightly acidic pH inhibits bacterial growth |
| Mucous membranes | Respiratory, digestive, urogenital tracts | Mucus traps pathogens and particles; cilia sweep mucus toward exits (mucociliary escalator); goblet cells continuously replenish mucus layer |
| Cilia | Respiratory tract lining | Coordinated beating moves mucus and trapped pathogens upward toward throat for swallowing or expulsion |
| Stomach acid | Stomach | HCl (pH 1.5–3.5) destroys most ingested pathogens before they reach the intestine |
| Lysozyme | Tears, saliva, nasal secretions | Enzyme that degrades peptidoglycan in bacterial cell walls — bactericidal in mucosal secretions |
| Normal microbiome | Skin, gut, vaginal tract | Commensal bacteria compete with pathogens for nutrients and attachment sites; some produce antimicrobial compounds |
| Sebaceous glands | Skin | Secrete sebum — slightly acidic oil that creates an inhospitable environment for many pathogens on skin surface |
When a pathogen breaches physical barriers and enters tissue, the inflammatory response begins within seconds. Its purpose is to deliver immune cells and chemical mediators to the site of infection, contain the pathogen, and begin repair.
The four cardinal signs of inflammation — redness, swelling, heat, and pain — are not random symptoms. Each has a specific cellular cause and a specific defensive purpose.
The four cardinal signs of inflammation — each caused by a specific cellular mechanism, each serving a defensive purpose
Inflammation does not just happen — it unfolds in a precise sequence triggered by chemical signals from damaged and infected cells.
The inflammatory cascade — from pathogen entry to immune cell recruitment and pathogen destruction
The inflammatory response is coordinated by chemical signals. Understanding what each does is essential for the HSC.
Fever is not just an unfortunate side effect of infection — it is a deliberate, adaptive response coordinated by the hypothalamus in response to chemical signals (primarily pyrogens like IL-1 and prostaglandins) from the immune system.
Misconception: Fever means the infection is winning — the body is overheating and failing.
Fever is an active defence mechanism initiated by the immune system, not a sign of failure. The hypothalamus deliberately raises the set point in response to pyrogens from immune cells. A moderate fever (37–38.5°C) speeds up immune reactions and may reduce pathogen replication efficiency. The body is not malfunctioning — it is running a programme that has been refined over hundreds of millions of years of evolution.
Misconception: Inflammation is always harmful — anti-inflammatory drugs should always be used to reduce it.
Inflammation is essential for pathogen clearance and tissue repair. The redness, swelling, and pain are signs that the immune system is working correctly. Suppressing inflammation with NSAIDs may reduce discomfort but can also delay healing and reduce immune effectiveness at the site. Anti-inflammatory treatment is appropriate when inflammation is excessive, prolonged, or causing more harm than benefit — not reflexively for every sign of infection.
Misconception: Pus is a sign that an infection is getting worse.
Pus is the product of a successful innate immune response — it consists largely of dead neutrophils that migrated to the infection site and destroyed pathogens. The formation and drainage of a pus-filled abscess is typically a sign that the immune system has contained and is eliminating the infection. A wound that is red, hot, swelling and spreading without pus formation may actually be more concerning — it may indicate the immune response is not effectively containing the infection.
Inflammatory Response — Step by Step
Activities
In your book, draw a cross-section diagram of skin and underlying tissue at a wound site 6 hours after infection by bacteria. Your diagram must include and label:
Type any notes or additional detail here after completing your diagram.
The table below shows data from an experiment investigating the effect of temperature on neutrophil phagocytosis rate and bacterial replication rate in vitro (in cell culture).
| Temperature (°C) | Neutrophil phagocytosis rate (bacteria/neutrophil/hour) | Bacterial replication rate (doublings/hour) |
|---|---|---|
| 36.0 (sub-normal) | 4.1 | 2.8 |
| 37.0 (normal body temp) | 5.2 | 2.9 |
| 38.0 (mild fever) | 6.8 | 2.7 |
| 38.5 (moderate fever) | 7.4 | 2.4 |
| 39.5 (high fever) | 7.1 | 2.1 |
| 40.5 (very high fever) | 5.3 | 1.8 |
| 41.5 (dangerous fever) | 2.9 | 1.5 |
Write your responses here or in your book.
You were asked to predict the cellular cause of each of the four signs of inflammation — redness, swelling, warmth, and pain.
The answers: redness and warmth share the same cause — vasodilation triggered by histamine from mast cells, which widens blood vessels and increases blood flow, bringing more warm blood close to the surface. Swelling has a different cause — increased capillary permeability (also triggered by histamine and other mediators) allows plasma to leak from blood vessels into the tissue space, causing fluid accumulation. Pain is caused by prostaglandins and bradykinin directly stimulating pain receptor nerve endings (nociceptors).
If you predicted that redness and swelling had the same cause — they are both triggered by histamine, but through different mechanisms (vasodilation vs permeability). If you predicted that warmth was caused by fever — local warmth at an infection site is due to increased blood flow, not systemic fever. Fever is a separate, whole-body response coordinated by the hypothalamus.
The key insight: every sign has a specific cellular mechanism and a specific defensive purpose. They are not random discomforts — they are information and action, happening simultaneously.
Assessment
5 random questions from a replayable lesson bank — feedback shown immediately
1. Explain how the skin and mucous membranes act as physical barriers against pathogen entry. For each, identify one structural feature and explain how it prevents infection. (3 marks)
1 mark: skin feature + mechanism | 1 mark: mucous membrane feature + mechanism | 1 mark: one additional feature of either (e.g. lysozyme, cilia, stomach acid, normal microbiome)
2. A student claims: "Inflammation is a harmful overreaction by the immune system — anti-inflammatory drugs should always be taken to reduce it." Evaluate this claim, referring to the causes and purposes of the four cardinal signs of inflammation. (3 marks)
1 mark: correctly identifies what causes the four signs | 1 mark: explains the defensive purpose of at least two signs | 1 mark: evaluative conclusion correctly assessing the student's claim
3. Describe the sequence of physical and chemical changes that occur in host animal tissue in the first 12 hours following bacterial infection of a skin wound. In your answer, refer to at least four specific mediators or cell types and explain the role of each. (4 marks)
1 mark per correctly described mediator/cell type with its role (max 4): mast cells/histamine, prostaglandins, cytokines/chemokines, neutrophils, complement, interferons
Answers
SA1: The skin acts as a physical barrier through its keratinised, multilayered epidermis — keratin makes the outer skin tough and relatively impermeable, and the constant shedding of dead outer cells removes any surface-dwelling microorganisms before they can establish infection. The skin's slightly acidic pH (around 4.5–5.5) also inhibits the growth of many pathogenic bacteria. Mucous membranes line the respiratory, digestive, and urogenital tracts — surfaces that cannot be covered by tough keratinised skin. Mucus itself physically traps pathogens and particles in a sticky gel layer. The coordinated beating of cilia lining the respiratory tract sweeps this mucus (with trapped pathogens) upward toward the throat in the mucociliary escalator — where it is swallowed and destroyed by stomach acid. An additional physical defence is lysozyme, present in tears, saliva, and nasal secretions — this enzyme degrades peptidoglycan in bacterial cell walls, killing bacteria on contact at mucosal surfaces before they can penetrate further.
SA2: The student's claim is incorrect. The four cardinal signs of inflammation — redness, swelling, heat, and pain — are each caused by specific cellular mechanisms that serve defined defensive purposes. Redness and heat are caused by vasodilation (triggered by histamine from mast cells), which increases blood flow to the infected area — this is the body delivering immune cells and chemical mediators rapidly to where they are needed. Swelling is caused by increased capillary permeability, allowing plasma (including antibodies and complement proteins) to flood the infection site. Pain, caused by prostaglandins and bradykinin stimulating nerve endings, serves to signal tissue damage and reduce use of the injured area, protecting it during repair. These responses are not harmful overreactions — they are coordinated, adaptive mechanisms essential for pathogen clearance and tissue repair. Anti-inflammatory drugs (such as NSAIDs) are appropriate when inflammation is excessive, prolonged, or causing more harm than benefit — but taking them reflexively suppresses responses that are actively assisting recovery, and may prolong infection clearance time. The claim is an oversimplification that misunderstands the function of inflammation.
SA3: Within seconds of bacterial entry through a skin wound, damaged tissue cells and mast cells in the surrounding connective tissue release histamine. Histamine causes vasodilation of local blood vessels — widening them to increase blood flow — and increases capillary permeability, allowing plasma to leak into the surrounding tissue. This produces the redness, warmth, and swelling of early inflammation, and delivers plasma proteins (including complement) to the site. Simultaneously, damaged cells release prostaglandins and bradykinin, which sensitise local pain receptors (nociceptors), causing the throbbing pain characteristic of infected wounds. Within the first hour, macrophages in the tissue begin phagocytosing bacteria and releasing cytokines — including interleukin-1 (IL-1), IL-6, and tumour necrosis factor alpha (TNF-α) — which travel via the bloodstream to the hypothalamus, triggering fever. These cytokines also act as chemokines, establishing a chemical concentration gradient that attracts neutrophils from the bloodstream toward the infection site in a process called chemotaxis. Within 2–6 hours, neutrophils migrate out of blood vessels through capillary walls (diapedesis) and begin engulfing bacteria by phagocytosis, killing them with reactive oxygen species and proteolytic enzymes. Dead neutrophils accumulate as pus. Complement proteins circulating in the blood are activated by bacterial surface molecules, coating bacteria (opsonisation) to facilitate phagocytosis and forming membrane attack complexes that directly punch holes in bacterial membranes. By 12 hours, if the infection is localised, this combination of physical, cellular, and chemical responses has typically contained the infection and begun repair processes.