Diseases and immunity
Pathogens and Transmission
LO 1: Defining a Pathogen
A pathogen is any disease-causing organism. While many students assume pathogens are only bacteria, they can also be viruses, fungi, or protoctists. For example, HIV is a virus, while Vibrio cholerae is a bacterium.
LO 2: Transmissible Diseases
A transmissible disease is one where the pathogen can be passed from one host (infected individual) to another. This distinguishes it from non-transmissible conditions like genetic disorders or lifestyle diseases (e.g., lung cancer from smoking).
LO 3: Modes of Transmission
Pathogens spread via two main routes:
- Direct contact: The pathogen passes directly from host to host. This includes physical touch, sexual contact, or through blood and other body fluids (e.g., HIV transmission via blood).
- Indirect transmission: The pathogen is passed via an intermediate vector or medium. Common examples include:
- Contaminated surfaces (fomites)
- Contaminated food or water
- Animals (vectors like mosquitoes for malaria)
- Air (droplets from coughing/sneezing)
⚠︎ Transmission Misconceptions
Error: Thinking that touching a contaminated surface is 'direct contact'.
Correction: Touching a contaminated surface is indirect transmission. Direct contact requires the pathogen to move directly from one living host to another (e.g., skin-to-skin or fluid-to-fluid).
Correction: Touching a contaminated surface is indirect transmission. Direct contact requires the pathogen to move directly from one living host to another (e.g., skin-to-skin or fluid-to-fluid).
Error: Believing all pathogens are bacteria.
Correction: Pathogens include viruses, fungi, and protoctists. Always identify the specific type if asked (e.g., Cholera is bacterial; Influenza is viral).
Correction: Pathogens include viruses, fungi, and protoctists. Always identify the specific type if asked (e.g., Cholera is bacterial; Influenza is viral).
Defining Transmissible Diseases
When to use: When asked to define or describe a transmissible disease.
Why examiners accept this: The definition must explicitly mention the pathogen and the transfer between hosts. Simply saying 'it spreads' is insufficient because it lacks the biological agent.
Correct phrasing: "A disease caused by a pathogen that is passed from one host to another."
Example: For HIV, you might specify: "A viral disease transmitted through direct contact with blood or body fluids."
Why examiners accept this: The definition must explicitly mention the pathogen and the transfer between hosts. Simply saying 'it spreads' is insufficient because it lacks the biological agent.
Correct phrasing: "A disease caused by a pathogen that is passed from one host to another."
Example: For HIV, you might specify: "A viral disease transmitted through direct contact with blood or body fluids."
Transmission Scenarios
Q:
State two ways a pathogen can be transmitted indirectly.
A:
- Through contaminated water.
- Via air (droplets from coughing).
Q:
Describe how HIV is transmitted directly.
A:
HIV is transmitted through direct contact with infected blood or other body fluids.
Body Defences
LO 4: Physical and Chemical Barriers
The body has several non-specific defences to prevent pathogens from entering:
- Skin: A physical barrier of tough, waterproof cells that prevents entry.
- Hairs in the nose: Trap dust and microbes from entering the respiratory tract.
- Mucus: Produced by epithelial cells in the trachea and bronchi; it traps pathogens.
- Ciliated cells (often linked with mucus): These have hair-like projections that beat to sweep mucus (with trapped pathogens) up towards the throat to be swallowed.
- Stomach acid: Highly acidic environment (low pH) that kills most bacteria ingested with food.
LO 4: White Blood Cells
If pathogens breach these barriers, white blood cells defend the body by:
- Phagocytosis: Engulfing and digesting pathogens.
- Producing antibodies: Proteins that target specific antigens on pathogens (see Immunity section).
Public Health Measures
LO 5: Controlling Disease Spread
Controlling transmission requires breaking the chain of infection:
- Clean water supply: Prevents waterborne diseases (e.g., Cholera) by removing pathogens.
- Hygienic food preparation: Cooking kills pathogens; washing hands prevents cross-contamination.
- Good personal hygiene: Washing hands removes pathogens from skin, preventing indirect transmission via surfaces or food.
- Waste disposal: Prevents accumulation of waste that attracts vectors (e.g., flies) which can carry pathogens to food.
- Sewage treatment: Removes harmful organisms/pathogens from wastewater before it is released back into the environment, preventing water contamination.
Explaining Public Health Measures
When to use: When asked to explain the importance of a measure like sewage treatment.
Why examiners accept this: You must link the action to the removal of pathogens and the prevention of spread. Avoid vague terms like 'keeping things clean'; be specific about the biological risk.
Correct phrasing: "Sewage treatment removes harmful organisms/pathogens from waste, preventing them from contaminating water supplies and causing bacterial infections."
Example: For hand washing: "Washing hands removes pathogens from the skin, preventing indirect transmission to food or surfaces."
Why examiners accept this: You must link the action to the removal of pathogens and the prevention of spread. Avoid vague terms like 'keeping things clean'; be specific about the biological risk.
Correct phrasing: "Sewage treatment removes harmful organisms/pathogens from waste, preventing them from contaminating water supplies and causing bacterial infections."
Example: For hand washing: "Washing hands removes pathogens from the skin, preventing indirect transmission to food or surfaces."
Body Defences and Hygiene
Q:
Describe three ways the body defends itself against infection.
A:
- Skin acts as a physical barrier.
- Mucus traps pathogens in the respiratory tract.
- Stomach acid kills ingested bacteria.
Q:
Explain why treating sewage is important for controlling disease.
A:
It removes harmful organisms/pathogens from waste, preventing them from contaminating water supplies and causing bacterial infections.
Active Immunity and Vaccination
LO 6 & LO 10: Active Immunity
Active immunity is defence against a pathogen by antibody production in the body. It is gained after:
Active immunity is defence against a pathogen by antibody production in the body. It is gained after:
- An infection by a pathogen (natural active immunity).
- Vaccination (artificial active immunity).
LO 7 & LO 9: Antigens and Antibodies
- Antigens: Molecules on the surface of pathogens. Each pathogen has its own unique antigens with specific shapes.
- Antibodies: Proteins produced by lymphocytes (a type of white blood cell). They have complementary shapes that fit specific antigens, like a key in a lock.
LO 8: Action of Antibodies
When antibodies bind to antigens:
- They can lead to the direct destruction of pathogens.
- They mark pathogens for destruction by phagocytes (a process called opsonization).
LO 11: The Process of Vaccination
- Weakened pathogens or their antigens are put into the body (usually via injection).
- These antigens stimulate an immune response by lymphocytes.
- Lymphocytes produce antibodies specific to those antigens.
- Memory cells are produced that give long-term immunity.
Note: The memory cells remain in the blood for years. If the real pathogen enters later, memory cells rapidly divide to produce large amounts of antibodies before the pathogen can cause significant harm.
⚠︎ Vaccination and Immunity Errors
Error: Believing that antibodies stimulate the immune response.
Correction: It is the antigens (from the pathogen/vaccine) that stimulate the immune response. Antibodies are the result of that response.
Correction: It is the antigens (from the pathogen/vaccine) that stimulate the immune response. Antibodies are the result of that response.
Error: Confusing antibiotics with antibodies.
Correction: Antibiotics are drugs used to kill bacteria. Antibodies are proteins produced by the body's immune system.
Error: Thinking vaccination produces immediate immunity.
Correction: Vaccination takes time to produce memory cells. Immediate protection is provided by passive immunity, not active vaccination.
Correction: Vaccination takes time to produce memory cells. Immediate protection is provided by passive immunity, not active vaccination.
Describing Active vs Passive Immunity
When to use: When asked to describe how immunity gained by vaccination differs from passive immunity.
Why examiners accept this: You must highlight that active immunity involves the body making its own antibodies and producing memory cells, whereas passive immunity involves receiving pre-made antibodies.
Correct phrasing: "Active immunity involves the person making their own antibodies and producing memory cells for long-term protection. Passive immunity involves acquiring antibodies from another individual, providing short-term protection without memory cells."
Example: For vaccination: "Vaccination introduces antigens to stimulate lymphocytes to produce antibodies and memory cells."
Why examiners accept this: You must highlight that active immunity involves the body making its own antibodies and producing memory cells, whereas passive immunity involves receiving pre-made antibodies.
Correct phrasing: "Active immunity involves the person making their own antibodies and producing memory cells for long-term protection. Passive immunity involves acquiring antibodies from another individual, providing short-term protection without memory cells."
Example: For vaccination: "Vaccination introduces antigens to stimulate lymphocytes to produce antibodies and memory cells."
Explaining the Role of Vaccination
When to use: When asked to explain the role of vaccination in controlling disease spread.
Why examiners accept this: You must link individual immunity to population-level effects (herd immunity).
Correct phrasing: "Vaccination reduces the number of susceptible hosts. This prevents the pathogen from spreading between people, thereby controlling the spread of the disease."
Example: "Mass vaccination creates herd immunity, protecting those who cannot be vaccinated by reducing person-to-person transmission."
Why examiners accept this: You must link individual immunity to population-level effects (herd immunity).
Correct phrasing: "Vaccination reduces the number of susceptible hosts. This prevents the pathogen from spreading between people, thereby controlling the spread of the disease."
Example: "Mass vaccination creates herd immunity, protecting those who cannot be vaccinated by reducing person-to-person transmission."
Vaccination Mechanism
Q:
Outline the process of vaccination.
A:
- Weakened pathogens or their antigens are put into the body.
- The antigens stimulate an immune response by lymphocytes which produce antibodies.
- Memory cells are produced that give long-term immunity.
Q:
Explain how vaccination controls the spread of disease.
A:
Vaccination reduces the number of people who can catch and spread the pathogen, thereby controlling the spread of the disease.
Passive Immunity
LO 13 & LO 15: Passive Immunity
Passive immunity is a short-term defence against a pathogen by antibodies acquired from another individual. The body does not produce its own antibodies or memory cells.
Passive immunity is a short-term defence against a pathogen by antibodies acquired from another individual. The body does not produce its own antibodies or memory cells.
Sources of Passive Immunity:
- Across the placenta: Mother passes antibodies to the fetus during pregnancy.
- Breast milk (Colostrum): Mother passes antibodies to the infant through breastfeeding.
LO 14: Importance of Breast-feeding
Breast-feeding is crucial because it provides the infant with passive immunity. Since infants' immune systems are not fully developed, these maternal antibodies protect them against infections during their early months of life until their own active immune system matures.
⚠︎ Passive vs Active Immunity Confusion
Error: Thinking memory cells are produced in passive immunity.
Correction: Memory cells are not produced in passive immunity because the body is not stimulated to mount its own immune response; it simply receives pre-made antibodies.
Correction: Memory cells are not produced in passive immunity because the body is not stimulated to mount its own immune response; it simply receives pre-made antibodies.
Describing Passive Immunity
When to use: When asked to explain passive immunity or the importance of breast-feeding.
Why examiners accept this: You must specify that antibodies are acquired from another individual and that it is short-term because no memory cells are formed.
Correct phrasing: "Passive immunity involves antibodies acquired from another individual, such as across the placenta or in breast milk. It provides short-term protection because memory cells are not produced."
Example: For breast-feeding: "Breast-feeding transfers maternal antibodies to the infant, providing passive immunity against pathogens until the infant's own immune system develops."
Why examiners accept this: You must specify that antibodies are acquired from another individual and that it is short-term because no memory cells are formed.
Correct phrasing: "Passive immunity involves antibodies acquired from another individual, such as across the placenta or in breast milk. It provides short-term protection because memory cells are not produced."
Example: For breast-feeding: "Breast-feeding transfers maternal antibodies to the infant, providing passive immunity against pathogens until the infant's own immune system develops."
Passive Immunity Details
Q:
Explain how infants gain passive immunity.
A:
Infants acquire antibodies from their mother across the placenta during pregnancy or through breast milk after birth.
Q:
State whether memory cells are produced in passive immunity.
A:
No, memory cells are not produced in passive immunity.
Cholera: A Case Study
LO 16: Cholera Pathogen
Cholera is a disease caused by a bacterium (Vibrio cholerae). It is transmitted in contaminated water (indirect transmission).
LO 17: Mechanism of Dehydration
The cholera bacterium produces a toxin that affects the small intestine:
- The toxin causes the secretion of chloride ions (Cl^-) into the lumen (inside) of the small intestine.
- This increases the solute concentration in the gut, lowering the water potential inside the intestine relative to the blood.
- Water moves from the blood into the gut by osmosis down the water potential gradient.
- This causes diarrhoea, leading to severe dehydration and loss of ions (electrolytes) from the blood.
⚠︎ Cholera Mechanism Errors
Error: Believing the cholera bacteria release chloride ions directly.
Correction: The toxin produced by the bacteria causes the cells to secrete chloride ions. The bacteria themselves do not contain or release the ions.
Correction: The toxin produced by the bacteria causes the cells to secrete chloride ions. The bacteria themselves do not contain or release the ions.
Error: Thinking the bacteria use up water in the body.
Correction: Water moves into the gut due to osmosis caused by ion concentration changes, not because the bacteria consume it.
Explaining Cholera Dehydration
When to use: When asked to explain how cholera causes dehydration (often a 3-4 mark question).
Why examiners accept this: You must follow the causal chain: Toxin → Chloride ions → Osmosis → Dehydration. Mentioning 'water potential' or 'concentration gradient' is key for full marks.
Correct phrasing: "The cholera bacterium produces a toxin that causes secretion of chloride ions into the small intestine. This increases the solute concentration, causing water to move from the blood into the gut by osmosis, leading to dehydration and loss of ions from the blood."
Example: Ensure you mention 'loss of ions from the blood' as a final consequence.
Why examiners accept this: You must follow the causal chain: Toxin → Chloride ions → Osmosis → Dehydration. Mentioning 'water potential' or 'concentration gradient' is key for full marks.
Correct phrasing: "The cholera bacterium produces a toxin that causes secretion of chloride ions into the small intestine. This increases the solute concentration, causing water to move from the blood into the gut by osmosis, leading to dehydration and loss of ions from the blood."
Example: Ensure you mention 'loss of ions from the blood' as a final consequence.
Cholera Details
Q:
Describe the type of pathogen causing cholera.
A:
A bacterium.
Q:
Explain how the cholera toxin causes dehydration.
A:
The cholera bacterium produces a toxin that causes secretion of chloride ions into the small intestine. This creates a water potential gradient, causing water to move from the blood into the gut by osmosis. This leads to diarrhoea, severe dehydration, and loss of ions from the blood.