Inquiry Question 2: How does a plant or animal respond to infection?
Investigate the innate and adaptive immune systems in mammals, including the response of animal innate immunity to infection (first and second lines of defence, including the inflammatory response)
A focused answer to the HSC Biology Module 7 dot point on innate (non-specific) immunity in animals. Covers the first line of defence (skin, mucous membranes, chemical barriers), the second line (phagocytosis, inflammation, natural killer cells, fever), and how these set up the adaptive response.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
NESA wants you to describe the two innate (non-specific) lines of defence in mammals, name the cells and chemicals involved, and explain the inflammatory response in detail. Innate immunity is examined every year in either multiple choice or short response.
Watch the command words. "Identify/state" wants the component and its job; "describe" wants the steps in order (especially the inflammatory cascade); "explain" wants the mechanism (how histamine produces each sign); and "evaluate" wants a justified judgement weighing innate against adaptive immunity. Marks are lost for vague verbs - say a phagocyte "engulfs" a pathogen, not that it "gets rid of" it.
The answer
The mammalian immune system has three layers. The first and second lines of defence are innate (non-specific), responding identically to any pathogen. The third line is adaptive (specific) and is covered in the next dot point. The diagram below traces what happens when a pathogen breaches each layer.
First line of defence: barriers
The first line prevents pathogens from entering the body. It is always active and requires no recognition. The figure below maps the main physical, chemical and biological barriers onto the body.
Physical barriers.
- Skin. A multilayered keratinised epidermis is the largest barrier. Continuous shedding of skin cells removes attached pathogens.
- Mucous membranes line the respiratory, digestive, urogenital and conjunctival tracts. Mucus traps pathogens; ciliated epithelium sweeps them out.
- Hair, nasal turbinates and eyelashes filter incoming air and debris.
Chemical barriers.
- Stomach acid (pH around 2) kills most ingested pathogens.
- Lysozyme in tears, saliva and sweat digests bacterial cell walls.
- Sebum on skin lowers pH and contains antimicrobial fatty acids.
- Antimicrobial peptides (defensins) puncture pathogen membranes.
Biological barriers.
- The normal microbiota on skin and in the gut outcompetes invading pathogens for nutrients and attachment sites.
Second line of defence: innate cellular response
If a pathogen breaches the first line, the second line activates within minutes to hours. It is still non-specific but now involves cells and signalling molecules.
Phagocytic cells.
- Neutrophils are the first responders. They migrate to the site within minutes and engulf pathogens.
- Macrophages arrive later and have higher capacity. They also present pathogen fragments to T cells, bridging to the adaptive response.
- Dendritic cells in tissue engulf pathogens and travel to lymph nodes to activate T cells.
Natural killer (NK) cells. Lymphocytes that recognise virus-infected and cancerous cells by their reduced MHC class I expression. They release perforin (forms pores in membranes) and granzymes (induce apoptosis).
Complement system. A cascade of around 30 plasma proteins that:
- Mark pathogens for phagocytosis (opsonisation).
- Recruit phagocytes (chemotaxis).
- Form a membrane attack complex (MAC) that lyses pathogen membranes.
Interferons. Cytokines released by virus-infected cells that signal neighbouring cells to enter an antiviral state, slowing viral spread.
The inflammatory response
Inflammation is the most visible part of the innate response. It has four cardinal signs: heat, redness, swelling and pain. The figure below follows the cascade from tissue damage to phagocytosis and fever.
Steps.
- Tissue damage. Pathogens or wounding trigger damaged cells and mast cells to release histamine, prostaglandins and bradykinin.
- Vasodilation. Local blood vessels widen, increasing blood flow (heat, redness).
- Increased capillary permeability. Plasma proteins and fluid leak into tissue, causing swelling (oedema).
- Chemotaxis. Cytokines attract neutrophils, then macrophages, to the site of damage.
- Phagocytosis. Pathogens are engulfed and destroyed.
- Resolution. Macrophages clear debris. Tissue repair begins.
If infection becomes systemic, cytokines (especially IL-1) act on the hypothalamus to raise the body's setpoint, producing fever. Mild fever enhances immune cell activity and slows pathogen growth.
How the innate response sets up adaptive immunity
The innate response is not a dead end - it actively switches on the adaptive (third) line. When a macrophage or dendritic cell phagocytoses a pathogen, it displays the pathogen's antigen fragments on its surface and becomes an antigen-presenting cell. Dendritic cells then travel to the lymph nodes and present that antigen to helper T cells, activating the specific response. This is why innate immunity is described as "buying time" - it both slows the pathogen and triggers the slower, specific defence.
Examples in context
Example 1. Splinter wound and the inflammatory response. A child running barefoot on a Coogee beach gets a splinter in the heel. Within minutes, mast cells in the surrounding dermis release histamine, dilating local blood vessels (vasodilation) and increasing capillary permeability. Blood plasma leaks into the tissue, producing the four classical signs of inflammation: heat (calor), redness (rubor), swelling (tumor) and pain (dolor). Neutrophils squeeze out of capillaries by diapedesis within 1 to 2 hours, attracted by chemokines released from damaged cells. They phagocytose bacteria from the splinter, with the resulting pus being a mixture of dead neutrophils, bacteria and tissue debris. The whole response is innate and operates identically every time, regardless of which bacterium is present.
Example 2. Mucociliary escalator in a Sydney commuter with a viral cold. Each breath drawn on a crowded Town Hall train platform carries roughly 10000 microbes into the airways. The respiratory epithelium's first line of defence is the mucociliary escalator: goblet cells secrete sticky mucus that traps inhaled particles, and ciliated epithelial cells beat in coordinated waves to sweep the mucus upward at roughly 1 cm per minute toward the pharynx, where it is swallowed and destroyed by stomach acid. Lysozyme in mucus also chemically degrades bacterial cell walls. Smoking paralyses cilia, which is why smokers cough more (manually clearing what cilia normally clear) and are more prone to respiratory infections like bronchitis.
Exam-style practice questions
Practice questions written in the style of NESA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2025 HSC2 marksA diagram shows components of the innate immune system in humans (nasal hair, tear glands, mucus lining, skin, stomach acid, urinary tract). State the role of TWO components that protect against infection.Show worked answer →
2 marks for stating how two named innate components protect against infection. Sample answers from the guidelines:
- Stomach acid – the acid inhibits the growth of, or kills, bacteria/pathogens.
- Skin – acts as a physical barrier to pathogen entry into the body.
Other acceptable components: nasal hair (traps particles), tear glands (wash away/contain antimicrobial lysozyme), mucus lining (traps pathogens), urinary tract (flushes pathogens out). One mark for some relevant information. Marker feedback: elaborate how each component prevents infection and use the components shown in the stimulus.
Source: NESA 2025 HSC Biology examination and marking guidelines.
2023 HSC2 marksDescribe how phagocytes help protect against pathogens.Show worked answer →
2 marks for a clear description of phagocyte action. Sample answer: phagocytes engulf or enclose a pathogen/antigen — they can identify non-self substances — and once the pathogen is engulfed, the phagocyte breaks it down/digests it. One mark for some relevant information. Marker feedback: use precise terminology such as "engulf" or "enclose" (and ideally that the pathogen is then destroyed); vague wording loses the second mark. Source: NESA 2023 HSC Biology examination and marking guidelines.
Practice questions
Original practice questions graded from foundation to exam level, each with a full worked solution. Try them before revealing the solution.
foundation2 marksDistinguish between the first and second lines of defence in the innate immune system.Show worked solution →
1 mark - first line. The first line of defence is a set of surface barriers (physical, chemical and biological) that prevent pathogens from entering the body - for example skin, mucous membranes, stomach acid and lysozyme. It acts before any pathogen gets into the tissues.
1 mark - second line. The second line is an internal cellular and chemical response that activates after a pathogen has breached the barriers - phagocytes, natural killer cells, the complement system, interferons and the inflammatory response.
The discriminator is "keeps pathogens out (barriers)" versus "deals with pathogens that got in (cells and inflammation)". Both are innate (non-specific) - an answer that calls the second line "specific" or "adaptive" loses a mark.
foundation3 marksIdentify three components of the first line of defence in humans and state how each protects against infection.Show worked solution →
Award 1 mark for each correctly named component paired with its protective function (any three):
- Skin
- A keratinised physical barrier that blocks pathogen entry; continuous shedding removes attached microbes.
- Mucous membranes / mucus
- Sticky mucus lining the airways and gut traps pathogens, and cilia sweep them out.
- Stomach acid (pH around 2)
- Kills or inhibits most ingested pathogens chemically.
Also accept: lysozyme in tears/saliva (digests bacterial cell walls), sebum (lowers skin pH), normal microbiota (competitive exclusion). Naming a component without its function earns nothing - the mark is for the protection mechanism.
foundation2 marksDescribe the role of phagocytes in the innate immune response.Show worked solution →
1 mark - engulf. Phagocytes (such as neutrophils and macrophages) recognise non-self material and engulf (enclose) the pathogen by phagocytosis, taking it into a vesicle (phagosome).
1 mark - destroy. The phagosome fuses with a lysosome and the pathogen is digested / broken down by enzymes.
Precise verbs matter: NESA rewards "engulf" or "enclose" and that the pathogen is then "destroyed/digested". Vague wording such as "the phagocyte gets rid of it" caps at 1 mark.
core4 marksDescribe the inflammatory response, naming the key chemical mediator and explaining how it produces the visible signs of inflammation.Show worked solution →
- 1 mark - trigger and mediator
- Tissue damage causes mast cells (and damaged cells) to release histamine (and other mediators such as prostaglandins).
- 1 mark - vasodilation
- Histamine causes local blood vessels to dilate (vasodilation), increasing blood flow and producing heat and redness.
- 1 mark - increased permeability
- Histamine also increases capillary permeability, so plasma and proteins leak into the tissue, causing swelling (oedema); pain arises from stretched tissue and prostaglandin sensitisation of pain receptors.
- 1 mark - cell recruitment
- Chemotaxis attracts phagocytes (neutrophils then macrophages) to the site, where they engulf and destroy pathogens.
A complete answer links histamine to BOTH vasodilation (heat/redness) and permeability (swelling), and names the cells recruited. Listing the four signs without the mechanism caps below full marks.
core3 marksExplain how the innate immune system, despite being non-specific, helps trigger the specific (adaptive) immune response.Show worked solution →
- 1 mark - antigen presentation
- Innate phagocytes - especially macrophages and dendritic cells - engulf a pathogen and display its antigen fragments on their surface (on MHC class II), becoming antigen-presenting cells.
- 1 mark - activation of adaptive cells
- Dendritic cells travel to lymph nodes and present the antigen to helper T cells, activating the specific response.
- 1 mark - link / signalling
- Cytokines released during inflammation also recruit and stimulate lymphocytes, so the fast innate response "buys time" and initiates the slower, specific adaptive response.
The marking hinges on the bridging idea: a non-specific cell (phagocyte/dendritic cell) presents a specific antigen that switches on adaptive immunity.
core4 marksA graph shows the neutrophil count at a wound site rising from 100 cells/mm³ at time 0 to 2500 at 6 hours, then declining to 800 by 24 hours. Describe the response over time and explain the rise and the fall.Show worked solution →
- 1 mark - describe the trend
- Neutrophils rise steeply over the first 6 hours (a roughly 25-fold increase to the peak of 2500), then fall to 800 by 24 hours - a rapid recruitment followed by a gradual decline.
- 1 mark - explain the rise
- During inflammation, histamine-driven vasodilation and increased permeability plus chemotaxis recruit neutrophils, which leave the capillaries (diapedesis) and migrate to the wound to phagocytose pathogens.
- 1 mark - explain the fall (cell fate)
- Neutrophils are short-lived; after phagocytosing pathogens they undergo apoptosis (die) - the dead neutrophils, bacteria and debris form pus.
- 1 mark - explain the fall (clearance)
- Macrophages arrive and clear the dead neutrophils and debris, so the count falls as the infection is controlled and resolution begins.
Full marks require both why numbers rise (recruitment) and why they fall (neutrophil apoptosis + macrophage clearance), tied to the figures given.
exam6 marksA child stands on a splinter at the beach. Within an hour the heel is red, warm, swollen and painful, and over the next day pus forms. Explain the innate immune events occurring at the wound, from the moment of injury to resolution.Show worked solution →
Target a sequenced response that moves through the inflammatory cascade and links each step to an observed sign. Award up to 6 marks across the stages below.
- Trigger (1 mark)
- The splinter damages tissue and introduces bacteria; damaged cells and mast cells release histamine (and prostaglandins, bradykinin).
- Vascular changes (1-2 marks)
- Histamine causes vasodilation (heat and redness) and increased capillary permeability, so plasma leaks into the tissue causing swelling; pain results from stretched tissue and prostaglandin sensitisation of nociceptors. This accounts for all four cardinal signs.
- Cell recruitment (1-2 marks)
- Chemotaxis draws neutrophils out of the capillaries (diapedesis) within 1-2 hours, followed by macrophages. They phagocytose the bacteria - engulfing and digesting them.
- Pus and resolution (1 mark)
- Spent neutrophils undergo apoptosis; the mixture of dead neutrophils, bacteria and debris is pus. Macrophages clear the debris and tissue repair begins.
- Non-specific framing (up to 1 mark, integrated)
- A Band 6 answer notes the response is innate / non-specific - identical regardless of the bacterium - and may note that macrophages presenting antigen can initiate the adaptive response. Marks are lost for an unordered list or for omitting the mediator (histamine) or the cells (neutrophils/macrophages).
exam7 marksEvaluate the importance of the innate immune system relative to the adaptive immune system in protecting a mammal from infectious disease.Show worked solution →
"Evaluate" requires a judgement supported by the strengths and limitations of each system. A Band 6 response reaches a reasoned conclusion, not just a comparison.
- What innate immunity offers (2 marks)
- It is fast (minutes to hours), broad (non-specific) and always ready - barriers (skin, mucus, stomach acid, lysozyme) stop most pathogens before entry, and the second line (phagocytes, NK cells, complement, inflammation, fever) attacks anything that breaches them. Crucially, it is the only defence active in the first days of a new infection and it initiates the adaptive response through antigen presentation.
- What adaptive immunity offers (2 marks)
- It is specific (targets a precise antigen), far more powerful against an established infection, and provides immunological memory, giving fast, strong protection on re-exposure (the basis of immunity and vaccination). Its limitation is that it is slow to start (days) and must be re-tailored to each pathogen.
- Weighing them (1-2 marks)
- Innate defence is indispensable - a person with a defective innate response is vulnerable to a wide range of pathogens immediately, whereas adaptive immunity, though decisive, depends on innate cells to be triggered and cannot act in time alone. Adaptive immunity, however, clears infections innate immunity cannot finish and prevents re-infection.
- Judgement (1 mark)
- A supported conclusion - for example: both are essential and interdependent, but the innate system is the more fundamental "first responder" because it acts immediately, defends against all pathogens, and is required to activate adaptive immunity; the adaptive system provides the specific, lasting protection the innate system cannot. An answer that describes both without an explicit, justified judgement caps below full marks.