Welcome to Disease and Body Defences!
Welcome to one of the most exciting topics in Biology! Every single day, your body is surrounded by millions of microscopic organisms trying to get inside. Yet, most of the time, you feel completely fine. How does your body do this? In this chapter, we will explore what causes disease, how your incredible immune system fights off invaders, how vaccinations protect us, and how modern medicines are developed.
Don't worry if this topic feels like it has a lot of new words — we will break down every single idea step-by-step with simple analogies to make revising easy!
1. Health and Types of Disease
Let's start with the basics: what does it actually mean to be "healthy"?
According to the World Health Organisation, health is a state of complete physical, mental, and social well-being, not just the absence of illness.
Communicable vs Non-Communicable Diseases
Diseases can be split into two main categories:
1. Communicable Diseases (Infectious):
• These are diseases that can be passed from one person to another.
• They are caused by tiny microscopic organisms called pathogens.
• Examples: The common cold, flu, chickenpox, COVID-19, and athlete's foot.
2. Non-Communicable Diseases (Non-Infectious):
• These are diseases that cannot be caught or passed from person to person.
• They are caused by lifestyle factors, genetics, or environmental issues.
• Examples: Heart disease, diabetes, cancer, and asthma.
What is a Pathogen?
A pathogen is a disease-causing microorganism. There are three main types you need to know for your exam:
• Bacteria: Tiny single-celled living organisms. Some bacteria release harmful poisons called toxins that make us feel ill. (Example: Salmonella food poisoning).
• Viruses: Even smaller than bacteria! Viruses are not made of cells. They enter our body cells, multiply inside them, and cause the cells to burst. (Example: Influenza / Flu virus, HIV).
• Fungi: Simple organisms that can be single-celled or multi-celled. (Example: Athlete's foot, ringworm).
How Do Communicable Diseases Spread?
Pathogens can enter the human body in several ways:
• Airborne / Droplets: Coughing or sneezing releases tiny liquid droplets carrying pathogens that others breathe in (e.g., flu, colds).
• Direct Contact: Touching an infected person or a contaminated surface (e.g., sharing a towel and getting athlete's foot).
• Contaminated Food or Water: Eating food that hasn't been cooked properly or drinking dirty water (e.g., food poisoning, cholera).
• Bodily Fluids: Transfer of blood or fluids during sexual intercourse or sharing needles (e.g., HIV).
Key Takeaway: Communicable diseases are spread by pathogens (bacteria, viruses, and fungi). Non-communicable diseases cannot be caught from someone else.
2. The First Line of Defence (Physical and Chemical Barriers)
Your body has fantastic natural security barriers designed to stop pathogens from getting inside in the first place.
Physical Barriers
• The Skin: Acts as an impermeable physical barrier covering the entire body. If the skin is cut, tiny cell fragments called platelets in the blood quickly form a scab. This plugs the wound to prevent blood loss and stop pathogens from entering.
• Mucus and Cilia in the Breathing System: The nose, trachea, and bronchi produce sticky mucus that traps dust and pathogens. Tiny hair-like structures called cilia then sweep the dirty mucus up to the back of the throat to be swallowed or spat out.
Chemical Barriers
• Stomach Acid: The stomach contains strong hydrochloric acid. This destroys most of the bacteria present in swallowed food and mucus.
• Tears and Saliva: Contain enzymes (such as lysozyme) that destroy the cell walls of certain bacteria.
Memory Trick: Think of your body like a medieval castle! The skin is the stone castle wall, cilia & mucus are the trapdoors, and stomach acid is the boiling moat.
Key Takeaway: The skin, mucus, cilia, and stomach acid are non-specific physical and chemical barriers that stop pathogens entering the bloodstream.
3. The Second Line of Defence (The Immune System)
If a pathogen manages to get past your physical barriers and enters your blood, your immune system takes over. This response is carried out by white blood cells.
There are two main types of white blood cells you must know:
1. Phagocytes (The "Eaters")
Phagocytes carry out a process called phagocytosis. This is a non-specific response, meaning they target any invader.
Step-by-step process of phagocytosis:
1. The phagocyte detects and moves towards the pathogen.
2. The phagocyte flows around the pathogen and engulfs it.
3. Digestive enzymes inside the phagocyte digest and destroy the pathogen.
2. Lymphocytes (The "Specialists")
Lymphocytes provide a specific defence by targeting specific pathogens.
• Every pathogen has unique chemical markers on its surface called antigens.
• Lymphocytes recognise these foreign antigens and produce special Y-shaped proteins called antibodies.
• Antibodies have a complementary shape that fits the antigen perfectly (just like a lock and key).
• When antibodies bind to the antigens, they cause the pathogens to stick together (clump), making it easy for phagocytes to engulf them.
• Lymphocytes also produce antitoxins to neutralise the harmful poisons (toxins) released by bacteria.
Memory Cells and Long-Term Immunity
When you encounter a pathogen for the first time, it takes a few days for your lymphocytes to make enough antibodies. During this time, you feel sick. This is the primary immune response.
After you recover, some of the lymphocytes remain in your blood as memory cells.
If the same pathogen enters your body again:
• The memory cells recognise the antigens immediately.
• They produce antibodies much faster and in much greater quantities.
• The pathogens are destroyed before you even feel unwell! This is the secondary immune response, and it means you are now immune.
Key Takeaway: Phagocytes engulf and digest pathogens. Lymphocytes produce specific antibodies that match antigens on the pathogen. Memory cells provide long-lasting immunity.
4. Vaccinations
What if we could give our body memory cells without having to get sick first? That is exactly how vaccines work!
How a Vaccine Works
1. A vaccine contains a dead, weakened, or inactive form of the pathogen (or just its antigens).
2. It is usually injected into the body.
3. The antigens in the vaccine stimulate your lymphocytes to produce specific antibodies.
4. Crucially, memory cells are produced and stay in the blood.
5. Because the pathogen was weakened or dead, you do not catch the actual disease.
6. If the live pathogen ever enters your body in the future, your memory cells quickly produce huge amounts of antibodies to wipe it out before you get sick.
Comparing Primary and Secondary Immune Responses
• Primary Response (First contact or vaccine): Slow to start, produces a lower concentration of antibodies, and antibody levels drop quickly.
• Secondary Response (Re-infection): Very rapid start, produces a much higher concentration of antibodies, and antibody levels remain high for a longer time.
Did You Know? Herd Immunity
When a large percentage of a population is vaccinated against a disease, it becomes very difficult for that disease to spread from person to person. This protects vulnerable people who cannot be vaccinated (such as very newborn babies or people with certain medical conditions). This community-wide protection is called herd immunity.
Key Takeaway: Vaccines introduce dead or weakened pathogens to trigger antibody and memory cell production, protecting you against future infections.
5. Medicines and Drug Development
When our body needs extra help fighting disease, we rely on medicines.
Antibiotics vs Painkillers
A common mistake in exams is confusing what different medicines do:
• Painkillers (e.g., paracetamol, ibuprofen): Relieve symptoms like pain, headache, or fever, but do not kill the pathogen.
• Antibiotics (e.g., penicillin): Medicines that kill bacteria or stop them from reproducing inside the body.
CRUCIAL EXAM POINT: Antibiotics ONLY kill bacteria. They have no effect on viruses (like the common cold or flu) because viruses live and reproduce inside human host cells.
Antibiotic Resistance
If antibiotics are overused, prescribed incorrectly, or if patients do not complete their full course of tablets, bacteria can mutate and become resistant.
• Resistant bacteria are not killed by the antibiotic.
• They survive, reproduce, and pass on their resistance.
• Bacteria that are resistant to multiple antibiotics are called "superbugs" (such as MRSA).
• To prevent resistance: doctors should only prescribe antibiotics when really necessary, and patients must always finish the entire prescribed course.
Testing and Developing New Drugs
Before a new medicine is given to the public, it must go through rigorous, strict testing to ensure it is safe, effective (does it work?), and to find the correct dosage.
Stage 1: Pre-clinical Testing
• Tested on cultured human cells and tissues in a laboratory.
• Tested on live animals (to check for toxicity and how the drug behaves in a whole living system).
Stage 2: Clinical Trials
• Phase 1: Tested on a small group of healthy human volunteers to check for safety and harmful side-effects.
• Phase 2 & 3: Tested on patients who have the disease to see if the drug actually works (efficacy) and to determine the ideal dose.
• Placebo and Blind Trials: In clinical trials, some patients are given a placebo (a fake tablet containing no active drug, like a sugar pill). In a double-blind trial, neither the doctor nor the patient knows who is receiving the real drug and who is receiving the placebo. This eliminates bias and psychological effects!
Key Takeaway: Antibiotics only kill bacteria, never viruses. New medicines undergo pre-clinical testing (cells, tissues, animals) followed by clinical trials (healthy volunteers, then patients) using placebos to ensure safety and effectiveness.
Quick Review Summary
• Health: Physical, mental, and social well-being.
• Pathogens: Microorganisms that cause communicable disease (Bacteria, Viruses, Fungi).
• Barriers: Skin (platelets/clotting), mucus, cilia, and stomach acid.
• White Blood Cells: Phagocytes engulf; Lymphocytes produce specific antibodies and memory cells.
• Vaccines: Use dead/weakened pathogens to create memory cells.
• Antibiotics: Kill bacteria, not viruses.
• Drug Testing: Laboratory tests (cells/animals) \(\rightarrow\) Healthy volunteers \(\rightarrow\) Ill patients (using double-blind trials).