Human defence systems (AQA GCSE Biology Combined Science): Revision Notes
Human defence systems
What are human defence systems?
The human body has non-specific defence systems that protect us from pathogens. These defence systems work all the time to stop harmful microorganisms from entering or damaging our body. You can find these defences in many places including:
- The nose, trachea and bronchi
- The stomach
- The skin
Non-specific defences are your body's first line of protection - they work against all types of pathogens, not just specific ones. This makes them incredibly important for keeping you healthy every day.
Chemical and physical defences
Your body uses two main types of defences to protect you from pathogens - chemical and physical barriers that work together to keep you safe.
Chemical defences
Chemical defences use special substances that can kill pathogens or stop them from working properly.
Lysozyme enzyme is found in:
- Tears - kills bacteria by breaking down their cell walls
- Saliva - protects your mouth from harmful bacteria
- Mucus - traps and destroys pathogens
Hydrochloric acid in your stomach:
- Kills pathogens that enter through food and drink
- Creates a very acidic environment that most harmful microorganisms cannot survive in
Physical barriers
Physical defences make it much harder for pathogens to get into your body in the first place.
Unbroken skin acts as a protective barrier because:
- It's too thick for most pathogens to get through
- It covers the whole outer surface of your body
Sticky mucus in the trachea and bronchi:
- Traps pathogens before they can reach your lungs
- Contains cilia (tiny hair-like structures) that move in a wave-like motion
- The cilia sweep mucus and trapped pathogens up towards your throat where you can swallow them
Physical and chemical defences work together as a team. For example, mucus (physical barrier) contains lysozyme (chemical defence) to both trap AND kill pathogens at the same time.
How skin protects you
Your skin is your body's largest organ and has several ways to defend against pathogens:
-
It secretes antimicrobial substances that kill pathogenic bacteria or stop them from growing
-
Non-pathogenic bacteria live on your skin - these "good" bacteria:
- Produce substances that kill harmful bacteria
- Compete with pathogenic bacteria for nutrients
- Take up space so harmful bacteria can't establish themselves
-
Scabs form over damaged skin to keep pathogens out while your skin repairs itself
Your skin isn't just a barrier - it's an active defence system! The "good" bacteria living on your skin are like friendly guards that help protect you from harmful invaders.
Real-world example: smoking and lung defences
Real-world Example: How Smoking Damages Your Defences
When people smoke cigarettes, the harmful substances can damage their lung defences. Here's what happens:
Step 1: The chemicals in cigarette smoke paralyse the cilia in the respiratory system
Step 2: This means the cilia can't move properly to sweep mucus and bacteria out of the lungs
Step 3: Mucus builds up in the lungs, and bacteria get trapped there
Result: This is why smokers develop a "smoker's cough" and are more likely to get lung infections
The cilia normally protect your lungs by moving mucus up and out, but smoking stops this important defence mechanism from working properly.
Key Points to Remember:
- Chemical defences use substances like lysozyme and stomach acid to kill pathogens
- Physical barriers like skin and mucus stop pathogens from entering your body
- Skin has multiple defence mechanisms including antimicrobial substances and helpful bacteria
- Cilia in your airways constantly sweep out mucus and trapped pathogens
- Smoking damages your natural lung defences by paralysing cilia