1. Overview
This topic explores how the human body protects itself against pathogens—organisms that cause disease. It covers the methods of disease transmission, the body's various levels of defense, and how medical interventions like vaccinations utilize our natural immune system to provide long-term protection.
Key Definitions
- Pathogen: A disease-causing organism.
- Transmissible Disease: A disease in which the pathogen can be passed from one host to another.
- Antigen: A chemical substance (usually a protein) found on the surface of a pathogen that has a specific shape and triggers an immune response.
- Antibody: A protein produced by lymphocytes that binds to specific antigens, leading to the destruction of pathogens.
- Active Immunity: Defence against a pathogen by antibody production in the body; gained after an infection or vaccination.
- Passive Immunity: Short-term defence against a pathogen by antibodies acquired from another individual (e.g., from mother to infant).
Core Content
Transmission of Pathogens
Pathogens can be passed from an infected person to an uninfected person in two main ways:
- Direct Contact: Through blood or other body fluids (e.g., HIV transmission via blood-to-blood contact or unprotected sex).
- Indirect Contact:
- Contaminated surfaces: Touching a doorknob or handle.
- Food: Eating undercooked meat or food handled with dirty hands.
- Animals: Mosquitoes (vectors) carrying malaria.
- Air: Breathing in droplets from sneezing or coughing (e.g., flu or common cold).
The Body’s Defences
The body has several mechanisms to prevent pathogens from entering or causing harm:
- Mechanical Barriers:
- Skin: A continuous outer layer that prevents pathogens from entering.
- Hairs in the nose: Traps large dust particles that may carry pathogens.
- Chemical Barriers:
- Mucus: Produced by lining of the airways to trap pathogens; then swept away by cilia.
- Stomach Acid (HCl): Kills most bacteria present in contaminated food or swallowed mucus.
- Cellular Barriers:
- White Blood Cells: These identify and destroy any pathogens that manage to breach the mechanical and chemical barriers.
A diagram showing a cross-section of the skin as a barrier, the respiratory tract with mucus and cilia, and the stomach containing gastric acid.
Controlling the Spread of Disease
Hygiene and infrastructure are vital for public health:
- Clean water supply: Prevents water-borne diseases like cholera.
- Hygienic food preparation: Washing hands, cooking meat thoroughly, and keeping raw/cooked foods separate.
- Good personal hygiene: Regular hand washing and showering.
- Waste disposal: Removing garbage that could attract flies or rats (vectors).
- Sewage treatment: Removing pathogens from human waste before the water is returned to the environment.
Extended Content (Extended Curriculum Only)
Active Immunity and Antibodies
Active immunity occurs when the body produces its own antibodies.
- Specific Defense: Every pathogen has unique antigens with specific shapes.
- Antibody Function: Lymphocytes produce antibodies with complementary shapes that fit specific antigens.
- Mechanism: Antibodies bind to antigens, which either:
- Directly destroy the pathogen.
- Mark the pathogen so phagocytes can easily find and ingest it.
Vaccination
Vaccination provides long-term immunity without the person having to get sick first.
- Process: Weakened or dead pathogens (or just their antigens) are introduced into the body.
- Immune Response: Lymphocytes recognize the antigens and produce specific antibodies.
- Memory Cells: The body produces memory cells. If the live pathogen enters the body later, these cells recognize it immediately and produce mass quantities of antibodies before symptoms develop.
Passive Immunity
Passive immunity is short-term because the body does not make its own antibodies and no memory cells are produced.
- Acquisition: Antibodies are passed from mother to baby across the placenta or through breast milk.
- Importance of Breast-feeding: It provides the infant with immediate protection against diseases while their own immune system is still developing.
Cholera: A Case Study
Cholera is a bacterium transmitted via contaminated water.
- Infection: Bacteria are ingested and reach the small intestine.
- Toxin Production: The bacteria produce a toxin.
- Ion Secretion: This toxin causes chloride ions (Cl⁻) to be secreted into the small intestine.
- Osmosis: The high concentration of ions lowers the water potential in the gut. Water moves out of the blood/cells and into the gut by osmosis.
- Result: Severe diarrhea, dehydration, and loss of essential ions from the blood.
Key Equations
While there are no specific formulas for this topic, you must understand the "Equation of Osmosis" in the context of Cholera:
- High solute concentration in gut + Low water potential in gut = Water movement into gut via osmosis.
Common Mistakes to Avoid
- ❌ Wrong: Antibiotics and Antibodies are the same thing.
- ✓ Right: Antibodies are proteins made by your immune system; Antibiotics are medicines used to kill bacteria.
- ❌ Wrong: Passive immunity lasts forever.
- ✓ Right: Passive immunity is temporary because the body does not produce memory cells for those antibodies.
- ❌ Wrong: Thinking the stomach "digests" pathogens.
- ✓ Right: The stomach acid kills pathogens by denaturing their enzymes/proteins.
Exam Tips
- Command Word "Explain": If asked to explain the cholera mechanism, you must mention chloride ions and osmosis to get full marks.
- Command Word "Suggest": Often used regarding why vaccination rates might drop or how a disease might spread in a disaster zone (e.g., lack of clean water).
- Specific Shapes: Always use the terms "antigen," "antibody," and "complementary shape" when discussing the immune response.
- Typical Context: Expect questions linking sewage treatment or clean water supply to the prevention of cholera outbreaks.
- Phagocytes vs Lymphocytes: Remember that Phagocytes engulf (phagocytosis) while Lymphocytes secrete (antibodies).