Human defence systems (AQA GCSE Biology): Revision Notes
Human defence systems
The human body has built-in defence systems that protect us from harmful microorganisms called pathogens. These defences work automatically and are found throughout our body, including in our nose, airways, stomach, and skin.
Chemical and physical defences
Our body uses two main types of defences to stop pathogens from causing infections.
Chemical defences
Chemical defences use special substances to kill pathogens or stop them from working properly.
Lysozyme enzyme is found in our tears and saliva. This enzyme breaks down the cell walls of bacteria, destroying them before they can cause harm. When you cry or produce saliva, you're actually helping to protect yourself from infection.
Lysozyme works by targeting the structural components of bacterial cell walls, making it an effective first line of chemical defence that we encounter daily through normal bodily functions.
Hydrochloric acid in the stomach is extremely strong and kills most pathogens that enter with our food and drink. The stomach's acidic environment acts like a chemical barrier, destroying harmful microorganisms before they can reach other parts of our body.
Physical barriers
Physical defences create obstacles that make it harder for pathogens to get inside our body.
Skin forms our most important physical barrier. It's thick and unbroken, making it very difficult for most pathogens to pass through. Think of your skin as natural armour protecting your internal organs.
Sticky mucus in the trachea and bronchi (airways) traps pathogens like a sticky trap. This mucus contains trapped bacteria and other harmful particles.
Cilia are tiny hair-like structures that line our airways. They move in wave-like motions, constantly sweeping mucus and trapped pathogens up towards the back of our throat where they can be swallowed and destroyed by stomach acid.
The coordinated movement of cilia creates a "mucociliary escalator" that continuously moves contaminated mucus away from the lungs, providing constant protection against respiratory infections.
Skin defences
Our skin does much more than just act as a physical barrier. It has several clever defence mechanisms that work together to provide comprehensive protection.
Antimicrobial protection
The skin produces antimicrobial substances that can kill pathogenic bacteria or stop them from growing. These natural chemicals work like invisible shields on our skin's surface.
The skin's antimicrobial properties are constantly active, providing 24/7 protection even when we're not aware of potential threats from pathogens in our environment.
Helpful bacteria
Many harmless bacteria live naturally on our skin. These non-pathogenic bacteria are actually helpful because they produce substances that kill harmful bacteria. They also compete with dangerous pathogens for nutrients, making it harder for infections to start.
This creates a beneficial relationship where our body provides a home for helpful bacteria, and they help protect us from harmful ones.
Scab formation
When our skin gets damaged, scabs quickly form over the wound. This keeps pathogens out while the skin underneath repairs itself. Scabs are like natural bandages that our body makes automatically.
How smoking affects defences
Cigarette smoke contains harmful substances that damage the cilia in our respiratory system. When cilia are paralysed, they can't sweep mucus and pathogens away properly. This is why smokers are more likely to get lung infections and develop a persistent cough as their lungs become clogged with mucus.
Smoking damage to cilia can be long-lasting and significantly increases vulnerability to respiratory infections. The paralysis of these tiny protective structures removes a crucial line of defence from the respiratory system.
Key Points to Remember:
- Non-specific defences protect against all types of pathogens, not just specific ones
- Chemical defences like lysozyme and stomach acid kill pathogens directly
- Physical barriers like skin and mucus prevent pathogens from entering the body
- Cilia act like tiny brooms, constantly cleaning our airways
- Skin produces antimicrobial substances and hosts helpful bacteria that fight infections
- Scabs form protective barriers over wounds while healing occurs