Second Line of Defence
In 2020, researchers at Monash University demonstrated that the 4 classic signs of inflammation, redness, swelling, heat, and pain, appear within 4 minutes of a wound forming, as the body's second line of defence fires automatically.
Printable Worksheets
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Q1 · What happens inside your body when you get a cut or a splinter? Describe what you think your body does to fight germs that get past the skin.
Q2 · A friend has a fever and wants to take medicine to bring it down immediately. What would you tell them about why their body might be heating up on purpose?
● Know
- The components of the second line of defence: inflammation, phagocytes, fever, complement proteins
- How each component works to fight pathogens
- That the second line of defence is non-specific and internal
● Understand
- Why inflammation is a protective response, not just a symptom
- How fever helps fight infection
- The role of phagocytes in destroying pathogens
● Can do
- Describe the inflammatory response
- Explain how phagocytes destroy pathogens
- Explain why a moderate fever can be beneficial
When you get a paper cut, your body responds within minutes. The area turns red, feels warm, swells up, and hurts. These four signs are the classic markers of inflammation. For centuries, doctors thought inflammation was a disease to be suppressed. We now know it is one of the most important protective mechanisms in biology.
Inflammation begins when damaged cells release chemical alarm signals into the surrounding tissue. These chemicals cause nearby blood vessels to dilate (widen), increasing blood flow to the area. More blood means more immune cells, more oxygen, and more nutrients for repair. The vessels also become more permeable, allowing fluid and proteins to leak into the tissue.
The swelling and pain are side effects of this protective process. Swelling occurs because fluid leaks from blood vessels into tissues. Pain occurs because swollen tissues press on nerve endings, and because inflammatory chemicals sensitise those nerves. The pain also has a protective function - it discourages you from using the injured area, preventing further damage.
When you sprain your ankle, it swells dramatically within hours. The swelling is not damage - it is your immune system rushing fluids, antibodies, and repair cells to the injured ligaments. Applying ice reduces swelling by constricting blood vessels, but it also slightly slows the delivery of immune cells. Doctors now recommend a balance: brief icing for pain relief, but not excessive icing that completely suppresses the inflammatory response.
Australian inflammation research: Scientists at the Walter and Eliza Hall Institute in Melbourne discovered how cytokines coordinate immune responses. Their work underpins treatments for inflammatory diseases like rheumatoid arthritis and Crohn disease, where inflammation becomes chronic and damaging instead of protective.
Fever and inflammation are always bad and should be suppressed immediately. This is wrong. While excessive inflammation can cause tissue damage, mild fever and inflammation are protective responses that speed up healing and fight infection. Suppressing a mild fever with paracetamol may make you more comfortable, but it can also slow your immune response.
When you get a cut, the area becomes red, swollen and warm. Most people think this is bad. But what if inflammation is actually helping you? Predict: what good things happen during inflammation?
How close was your prediction?
Nice calibration, your intuition is good for this kind of problem.
Good, being surprised is the point. This answer is worth remembering.
The stars of the second line of defence are phagocytes - white blood cells that specialise in eating invaders. The word comes from Greek phagein (to eat) and kytos (cell). These cells patrol your blood and tissues, constantly searching for foreign particles to engulf and destroy.
Neutrophils are the most abundant white blood cells and the first to arrive at an infection site. They live for only a few days, but they are ferocious eaters. A single neutrophil can engulf dozens of bacteria before dying. When neutrophils die in large numbers, they form pus - the white or yellow fluid you see in infected wounds.
Macrophages are larger, longer-lived phagocytes that arrive after neutrophils. Their name means big eater. They not only engulf pathogens but also clean up dead cells and debris. Crucially, macrophages act as messengers between the innate and adaptive immune systems. After eating a pathogen, they display fragments of it on their surface so T cells can recognise the invader.
When bacteria enter a cut, neutrophils arrive within minutes to hours. They release chemicals that kill the bacteria but also damage nearby tissue. After 24-48 hours, macrophages take over. They eat the remaining bacteria, dead neutrophils, and damaged tissue cells. Then they release growth factors that stimulate tissue repair. Without macrophages, wounds would heal slowly and incompletely.
Australian biomedical research: Researchers at Monash University study how macrophages switch between pro-inflammatory and pro-repair modes. Understanding this switch could lead to treatments that accelerate wound healing in diabetic patients.
Pus means the infection is getting worse. Pus is actually a sign that your immune system is fighting effectively. It is mostly dead neutrophils, bacteria, and tissue fluid. While a lot of pus may indicate a heavy bacterial load, the presence of pus itself means your phagocytes are actively killing invaders.
- Neutrophil
- Macrophage
- Phagosome
- Opsonisation
- Coating pathogens to make them easier to engulf
- Most abundant; first responder
- Larger; cleans up debris and presents antigens
- Bubble inside cell that encloses the pathogen
Fever is one of the most misunderstood symptoms in medicine. For centuries, doctors treated fever as the enemy, using cold baths and drugs to bring body temperature down. We now understand that fever is a deliberate immune strategy - your body raises its temperature on purpose to fight infection more effectively.
Many bacteria and viruses replicate most efficiently at normal body temperature (37C). When your thermostat rises to 38-39C, their reproduction slows dramatically. At the same time, your immune cells become more active. Phagocytes move faster, antibodies are produced more quickly, and interferon works more effectively.
However, fever has limits. Very high temperatures (above 40-41C) can damage proteins and organs, particularly in young children. Infants under three months with any fever need immediate medical attention. For older children and adults, mild to moderate fever is usually best left alone, or treated only for comfort.
When you have influenza, your body temperature rises to 38-39C. This fever makes it harder for the influenza virus to replicate in your respiratory tract. Your phagocytes also become more aggressive. If you immediately take paracetamol to suppress the fever, you may feel more comfortable, but you might also prolong the infection by removing one of your body natural defences.
Australian paediatric guidelines: The Royal Children Hospital Melbourne advises that fever in otherwise healthy children is usually harmless and does not require medication unless the child is distressed. Their guidelines emphasise hydration and comfort over aggressive fever suppression.
If I have a fever, I need antibiotics. Fever is a symptom, not a disease. It can be caused by viruses, bacteria, fungi, or even non-infectious conditions. Antibiotics only treat bacterial infections. Taking antibiotics for a viral fever is ineffective and contributes to antibiotic resistance.
While phagocytes and inflammation get most of the attention, the immune system has a hidden ally: the complement system. This is a group of roughly 30 proteins that circulate in your blood in an inactive state. When they encounter a pathogen, they activate in a cascade and unleash multiple attacks simultaneously.
The complement system has three main functions:
- Direct destruction - some complement proteins punch holes in bacterial cell membranes, causing the bacteria to burst. This is called the membrane attack complex.
- Tagging for destruction - complement proteins coat pathogens in a process called opsonisation, making them much easier for phagocytes to recognise and engulf. A tagged bacterium is eaten 100 times faster than an untagged one.
- Recruitment - complement fragments attract more immune cells to the infection site and promote inflammation.
The complement system bridges the second and third lines of defence. It enhances phagocytosis and helps antibodies work more effectively.
People with a rare genetic disorder called C3 deficiency lack a key complement protein. They suffer from severe, recurrent bacterial infections starting in infancy. Even though they have normal numbers of phagocytes and antibodies, those cells cannot work effectively without complement to tag bacteria and punch holes in them.
Australian immunology: Researchers at the Centenary Institute in Sydney study how complement proteins contribute to inflammatory diseases like atherosclerosis. In these conditions, complement becomes overactive and damages healthy tissue.
Complement is just a helper system, not essential. This is false. Complement is absolutely essential for effective immunity. Without it, phagocytes are blind and slow, antibodies are weak, and bacterial infections become deadly.
The system is a group of about 30 proteins that help fight infection by punching holes in , marking them for destruction through , attracting immune cells, and promoting .
Wrong: "Inflammation means the infection is getting worse." Not necessarily, inflammation is a protective response. Redness and swelling indicate that immune cells are arriving to fight the infection. However, excessive inflammation can cause tissue damage.
Right: Inflammation is a protective immune response. Redness, heat, swelling, and pain indicate that immune cells, oxygen, and nutrients are being delivered to fight the infection and begin repair.
Wrong: "Fever should always be reduced immediately." Not always, moderate fever (38-39°C) is beneficial because it slows pathogen growth and enhances immune responses. Fever-reducing medication is usually only needed for comfort or if the fever is very high.
Right: Moderate fever is a beneficial defence mechanism. It slows pathogen reproduction and enhances immune cell activity, so it should usually be left alone unless the temperature is very high or the person is distressed.
Wrong: "Pus is a sign that the body is losing the fight." No, pus is actually a collection of dead phagocytes, pathogens, and tissue debris. It shows that phagocytes have been actively fighting and destroying invaders.
Right: Pus is mostly dead neutrophils, destroyed pathogens, and tissue debris, clear evidence that phagocytes are actively fighting and winning the battle against invaders.
Fever in Australian Aboriginal Culture
Traditional healing practices: Aboriginal and Torres Strait Islander cultures have long recognised fever as a sign that the body is fighting illness. Traditional healers used bush medicines with antipyretic (fever-reducing) properties, such as extracts from the gumby gumby tree (Pittosporum phylliraeoides), to help manage fever while supporting the body's natural healing.
Rheumatic fever in Indigenous communities: Acute rheumatic fever is an inflammatory disease that can develop after untreated streptococcal throat infections. It is now rare in most developed countries but remains a significant problem in remote Aboriginal and Torres Strait Islander communities in northern Australia. If untreated, it can lead to rheumatic heart disease, which causes lifelong damage to heart valves. Australia has established rheumatic fever registers to track cases and guide prevention programs, including regular antibiotic injections for at-risk individuals.
Fever management in modern Australia: The Royal Children's Hospital Melbourne advises that fever in children is a normal immune response and that the primary goal is keeping the child comfortable rather than reducing temperature to normal. This reflects modern understanding that moderate fever is protective.
✍ Copy Into Your Books
▾Inflammation
- Redness, heat, swelling, pain
- Increases blood flow to infection site
- Attracts immune cells
- Seals off infected area
Phagocytes
- Neutrophils: first responders, engulf pathogens
- Macrophages: larger, clean up debris
- Phagocytosis: surround, engulf, digest
Fever and Complement
- Fever slows pathogen growth, enhances immunity
- Complement proteins: punch holes, mark pathogens, attract cells
Defence Diagrams
Phagocyte Investigation
At the start of this lesson, you thought about getting a splinter, the skin turns red, swells, and heats up within minutes, and asked why your immune system would deliberately cause inflammation and fever to fight an infection.
Now that you've studied the second line of defence, can you explain why inflammation is actually a protective response and not just damage? Has your view of fever and swelling changed?
Q1. 1. Describe the four main components of the second line of defence and explain how each helps fight infection. 4 MARKS
Q2. 2. Explain why inflammation is a protective response rather than simply a symptom of disease. Include the benefits of each of the four classic signs. 4 MARKS
Q3. 3. A patient has a genetic disorder that prevents their complement proteins from working. Predict how this would affect their ability to fight bacterial infections and explain your reasoning. 4 MARKS
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- Can you now explain how your body fights pathogens that get past the skin and mucous membranes?
- Why are redness, swelling, and fever signs that your body is working to protect you?
Model answers (click to reveal)
Answers
▾MCQ 1
CPhagocytes are part of the second line of defence. Skin and mucous membranes are first-line defences. Antibodies are part of the third line.
MCQ 2
ADuring phagocytosis, a phagocyte surrounds the pathogen with its cell membrane, engulfs it into a phagosome, and digests it with enzymes.
MCQ 3
BModerate fever (38-39°C) slows the reproduction of many pathogens and enhances the activity of immune cells like phagocytes.
MCQ 4
BComplement proteins punch holes in pathogen membranes and coat pathogens to make them easier for phagocytes to recognise and destroy.
MCQ 5
BNeutrophils are the most abundant phagocytes and are the first to arrive at infection sites, usually within hours.
Short Answer 1
Model answer: The four main components of the second line of defence are: (1) Inflammationincreased blood flow brings immune cells, oxygen, and nutrients to infection sites. Blood vessels become leaky, allowing fluid and white blood cells to enter tissues. (2) Phagocytesneutrophils and macrophages engulf and digest pathogens through phagocytosis. Neutrophils arrive first; macrophages clean up later. (3) Feverelevated body temperature slows pathogen reproduction and enhances the activity of immune cells. (4) Complement proteinsthese blood proteins punch holes in pathogen membranes, mark pathogens for phagocytosis, and attract immune cells to infection sites.
Short Answer 2
Model answer: Inflammation is a protective response because each of its signs serves a defensive purpose. Redness and heat result from increased blood flow, which delivers more oxygen, nutrients, and immune cells to the infected area. Swelling occurs because blood vessels become more permeable, allowing fluid and white blood cells to leak into tissues; this helps contain the infection and prevents it from spreading. Pain alerts the person to protect the affected area and rest, which aids recovery. While inflammation can be uncomfortable, it is evidence that the immune system is actively fighting the infection.
Short Answer 3
Model answer: A patient without functioning complement proteins would be severely impaired in fighting bacterial infections. Complement proteins destroy bacteria directly by punching holes in their membranes, causing them to burst. Without this mechanism, bacteria would survive longer. Additionally, complement proteins coat bacteria (opsonisation), making them easier for phagocytes to recognise and engulf. Without opsonisation, phagocytosis would be less efficient. Complement also attracts neutrophils and macrophages to infection sites through chemical signals. Without these signals, immune cells would arrive more slowly. As a result, bacterial infections would be more severe, last longer, and be more likely to spread. The patient would likely need more aggressive antibiotic treatment and might experience recurrent infections.