Reproduction, Immunity, and Health
3. The Immune System
Learning outcomes
- I can describe the function of the immune system.
- I can identify the body's first, second, and third lines of defense against disease.
- I can explain how white blood cells help protect the body from pathogens.
- I can distinguish between innate immunity and adaptive immunity.
- I can explain how the immune system helps maintain health.
Introduction
Every day, your body is exposed to millions of microorganisms. Most are harmless, but some—called pathogens—can cause disease.
Fortunately, your body has a remarkable defence system that works around the clock to protect you from infection. This is called the immune system.
The immune system is made up of specialized cells, tissues, organs, and proteins that work together to detect, destroy, and remember harmful pathogens. Without it, even a minor infection could become life-threatening.
What Is the Immune System?
The immune system is the body's defence system against disease-causing organisms.
Its main functions are to:
- Detect harmful pathogens.
- Prevent pathogens from entering the body.
- Destroy invading microorganisms.
- Remove infected or damaged cells.
- Remember previous infections for faster protection in the future.
The immune system works continuously, even while we sleep.
Pathogens
A pathogen is a microorganism that causes disease.
Common types of pathogens include:
- Bacteria
- Viruses
- Fungi
- Protists
- Parasitic worms
Not all microorganisms are harmful. Many bacteria, for example, live in our digestive system and help us stay healthy.
The Three Lines of Defence
The immune system protects the body using three lines of defence.
- First Line of Defence – Physical and chemical barriers that prevent pathogens from entering.
- Second Line of Defence – Fast, non-specific internal responses.
- Third Line of Defence – Specific immune responses that target particular pathogens.
Each line provides an additional level of protection.
First Line of Defence
The first line of defence prevents pathogens from entering the body.
Examples include:
| Defence | Function |
|---|---|
| Skin | Forms a tough physical barrier. |
| Mucus | Traps microorganisms in the nose and airways. |
| Tiny hairs (cilia) | Sweep mucus and trapped pathogens out of the respiratory system. |
| Tears | Wash away microorganisms and contain antibacterial enzymes. |
| Saliva | Helps destroy bacteria and keeps the mouth clean. |
| Stomach acid | Kills many microorganisms that enter with food. |
These barriers protect us before pathogens can cause infection.
Second Line of Defence
If pathogens enter the body, the second line of defence responds quickly.
This response is non-specific, meaning it attacks many different pathogens in the same way.
Important components include:
- White blood cells called phagocytes.
- Inflammation.
- Fever.
- Antimicrobial proteins.
Inflammation
Inflammation is one of the body's first responses to injury or infection.
Signs of inflammation include:
- Redness
- Swelling
- Heat
- Pain
Inflammation increases blood flow to the affected area, bringing more immune cells to fight infection.
Fever
A fever is an increase in body temperature.
A moderate fever can help by:
- Slowing the growth of some pathogens.
- Helping immune cells work more effectively.
Fever is a normal part of many immune responses, although very high fevers require medical attention.
White Blood Cells
White blood cells (leukocytes) are the main cells of the immune system.
Different types perform different jobs.
Examples include:
- Phagocytes – engulf and digest pathogens.
- Lymphocytes – recognize specific pathogens and produce immune responses.
Phagocytosis
A phagocyte destroys pathogens through a process called phagocytosis.
Steps include:
- The pathogen is recognized.
- The phagocyte surrounds it.
- The pathogen is enclosed inside the cell.
- Digestive enzymes break it down.
- The waste is removed.
Phagocytosis helps prevent infections from spreading.
Third Line of Defence
The third line of defence is the adaptive immune system.
Unlike the second line, it is specific.
It recognizes individual pathogens and produces targeted responses.
Key cells include:
- B lymphocytes (B cells)
- T lymphocytes (T cells)
Antibodies
B cells produce proteins called antibodies.
Antibodies:
- Attach to specific pathogens.
- Mark pathogens for destruction.
- Help prevent pathogens from infecting cells.
Each antibody fits only one specific pathogen, much like a key fits a lock.
T Cells
T cells perform several important functions.
Some T cells:
- Destroy infected body cells.
Others:
- Coordinate the activities of other immune cells.
Together, B cells and T cells form the body's highly specialized defence system.
Immune Memory
One of the greatest advantages of adaptive immunity is memory.
After recovering from certain infections:
- Memory cells remain in the body.
- They recognize the same pathogen if it returns.
- The immune response becomes much faster and stronger.
This is why people often do not become sick from the same disease twice.
Memory cells are also the reason vaccines provide long-term protection.
Innate vs Adaptive Immunity
| Innate Immunity | Adaptive Immunity |
|---|---|
| Present from birth | Develops after exposure to pathogens |
| Fast response | Slower first response |
| Non-specific | Highly specific |
| No long-term memory | Produces immune memory |
| Includes skin, inflammation, phagocytes | Includes B cells, T cells, antibodies |
Both systems work together to protect the body.
How the Immune System Maintains Health
The immune system helps us stay healthy by:
- Preventing infections.
- Destroying harmful microorganisms.
- Removing infected cells.
- Remembering previous infections.
- Supporting recovery after illness.
A healthy immune system allows the body to respond quickly to disease-causing organisms.
Real-World Applications
Understanding the immune system is important for:
- Vaccination.
- Preventing infectious diseases.
- Developing new medicines.
- Organ transplantation.
- Cancer immunotherapy.
- Public health.
Knowledge of immunity has saved millions of lives through improved disease prevention and treatment.
Worked Examples
Example 1
What is the main function of the immune system?
Answer:
To protect the body from pathogens and disease.
Example 2
Which line of defence includes the skin?
Answer:
The first line of defence.
Example 3
What type of white blood cell engulfs pathogens?
Answer:
A phagocyte.
Example 4
Which cells produce antibodies?
Answer:
B lymphocytes (B cells).
Example 5
What is one major difference between innate and adaptive immunity?
Answer:
Innate immunity responds quickly but is non-specific, whereas adaptive immunity is highly specific and develops immune memory.
Did You Know?
Your body can produce billions of different antibodies, allowing the immune system to recognize and respond to an enormous variety of pathogens. This incredible diversity helps protect you from diseases you have never encountered before.
Key Terms
| Term | Definition |
|---|---|
| Immune System | The body's defence system against disease-causing organisms. |
| Pathogen | A microorganism that causes disease. |
| Innate Immunity | The body's non-specific defence system that is present from birth. |
| Adaptive Immunity | The body's specific defence system that develops after exposure to pathogens. |
| White Blood Cell (Leukocyte) | An immune cell that helps defend the body against infection. |
| Phagocyte | A white blood cell that engulfs and digests pathogens. |
| Lymphocyte | A white blood cell involved in adaptive immunity, including B cells and T cells. |
| Antibody | A protein produced by B cells that binds specifically to a pathogen. |
| Inflammation | A protective response to injury or infection that increases blood flow and immune activity. |
| Immune Memory | The ability of the adaptive immune system to respond more quickly to pathogens it has encountered before. |
Key Takeaways
- The immune system protects the body against pathogens and disease.
- The first line of defence includes physical and chemical barriers such as the skin, mucus, tears, and stomach acid.
- The second line of defence provides rapid, non-specific protection through inflammation, fever, and phagocytic white blood cells.
- The third line of defence uses B cells, T cells, and antibodies to target specific pathogens and create long-lasting immune memory.
- Innate immunity provides immediate protection, while adaptive immunity is highly specific and remembers previous infections.
- A healthy immune system is essential for preventing disease, fighting infections, and maintaining overall health.