When a splinter pierces your skin, the area turns red, swells, and feels warm. These familiar signs mean your second line of defence has activated. Inflammation, fever, and special cells called phagocytes work together to contain and destroy invaders before they can spread.
Think about the last time you had a small cut that became red and swollen around the edges.
Write down your answers before reading on:
The body's alarm system
When tissues are damaged or infected, the body triggers inflammation — the familiar redness, heat, swelling, and pain that surround a wound or infection.
Inflammation serves important protective functions:
The four classic signs of inflammation were described by Roman physicians over 2,000 years ago: rubor (redness), calor (heat), tumor (swelling), and dolor (pain). These are not symptoms of disease — they are signs that your body is fighting back.
Engulfing and destroying invaders
Phagocytes are white blood cells that patrol the body, seeking and destroying pathogens. The two main types are:
The process of phagocytosis works like this:
A single neutrophil can engulf and destroy 5-20 bacteria before it dies.
When turning up the heat saves your life
Fever — an increase in body temperature above the normal 37°C — is one of the most powerful and ancient defences against infection.
Fever helps fight infection in several ways:
However, very high fevers (above 40°C) can be dangerous, especially for young children, because they can damage proteins and tissues. This is why medical advice is sought for very high fevers.
The molecular reinforcements
The complement system is a group of about 30 proteins circulating in the blood. When activated, they work together like a cascade of dominoes to destroy pathogens.
Complement proteins help fight infection by:
The complement system bridges the second and third lines of defence. While it is non-specific, it also helps prepare and enhance the specific immune response.
"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.
"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.
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.
1. Which of the following is part of the second line of defence?
2. What happens during phagocytosis?
3. Why can a moderate fever be beneficial during an infection?
4. Complement proteins help fight infection by:
5. Which white blood cell arrives first at an infection site?
1. Describe the four main components of the second line of defence and explain how each helps fight infection. 4 MARKS
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
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
Go back to your Think First answer. Has your understanding changed?
C — Phagocytes are part of the second line of defence. Skin and mucous membranes are first-line defences. Antibodies are part of the third line.
A — During phagocytosis, a phagocyte surrounds the pathogen with its cell membrane, engulfs it into a phagosome, and digests it with enzymes.
B — Moderate fever (38-39°C) slows the reproduction of many pathogens and enhances the activity of immune cells like phagocytes.
B — Complement proteins punch holes in pathogen membranes and coat pathogens to make them easier for phagocytes to recognise and destroy.
B — Neutrophils are the most abundant phagocytes and are the first to arrive at infection sites, usually within hours.
Model answer: The four main components of the second line of defence are: (1) Inflammation — increased 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) Phagocytes — neutrophils and macrophages engulf and digest pathogens through phagocytosis. Neutrophils arrive first; macrophages clean up later. (3) Fever — elevated body temperature slows pathogen reproduction and enhances the activity of immune cells. (4) Complement proteins — these blood proteins punch holes in pathogen membranes, mark pathogens for phagocytosis, and attract immune cells to infection sites.
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.
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.
Command phagocytes and trigger inflammation to destroy invading pathogens! Manage fever and complement proteins to win the battle.
Tick when you have finished all activities and checked your answers.