Diseases and immunity

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.

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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).

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Core Content

Transmission of Pathogens

Pathogens can be passed from an infected person to an uninfected person in two main ways:

1. Direct Contact: Through blood or other body fluids (e.g., HIV transmission via blood-to-blood contact or unprotected sex).
2. 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.

Controlling the Spread of Disease

Hygiene and infrastructure are vital for public health:

1. Clean water supply: Prevents water-borne diseases like cholera.
2. Hygienic food preparation: Washing hands, cooking meat thoroughly, and keeping raw/cooked foods separate.
3. Good personal hygiene: Regular hand washing and showering.
4. Waste disposal: Removing garbage that could attract flies or rats (vectors).
5. Sewage treatment: Removing pathogens from human waste before the water is returned to the environment.

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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:
  1. Directly destroy the pathogen.
  2. 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.

1. Process: Weakened or dead pathogens (or just their antigens) are introduced into the body.
2. Immune Response: Lymphocytes recognize the antigens and produce specific antibodies.
3. 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.

1. Infection: Bacteria are ingested and reach the small intestine.
2. Toxin Production: The bacteria produce a toxin.
3. Ion Secretion: This toxin causes chloride ions (Cl⁻) to be secreted into the small intestine.
4. 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.
5. Result: Severe diarrhea, dehydration, and loss of essential ions from the blood.

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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.

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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.

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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).

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0610 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

Cholera is a disease caused by the bacterium Vibrio cholerae. It is often transmitted through contaminated water.

(a) State two methods of indirect transmission of pathogens, other than contaminated water. [2]

(b) State three ways to help control the spread of cholera in an area with a recent outbreak. [3]

Worked Solution:

(a)

1. Contaminated food. A clear statement of a method of indirect transmission.
2. Animals (vectors), such as flies. A second clear statement of a method of indirect transmission.

How to earn full marks:

• One mark for each correctly identified method of indirect transmission.
• Do not accept 'direct contact' or any method involving direct contact.

(b)

1. Ensure a clean water supply through boiling or chlorination. A clear statement of an appropriate measure.
2. Provide adequate sewage treatment to prevent contamination of water sources. A second clear statement of an appropriate measure.
3. Promote good personal hygiene, such as regular handwashing with soap. A third clear statement of an appropriate measure.

How to earn full marks:

• One mark for each correctly identified method to control the spread of cholera.
• Accept other reasonable answers related to hygiene and sanitation.

Common Pitfall: Many students confuse direct and indirect transmission routes. Remember that indirect transmission involves an intermediate object or organism, while direct transmission involves physical contact between individuals. Also, be specific with your control measures; simply stating "hygiene" is not enough – you need to specify what hygienic practices are important.

Exam-Style Question 2 — Short Answer [6 marks]

Question:

The human body has several natural defense mechanisms against pathogens.

(a) State two functions of mucus in the human body as a defense against pathogens. [2]

(b) Describe how white blood cells defend the body against pathogens. [4]

Worked Solution:

(a)

1. Trapping pathogens (e.g., bacteria, viruses) to prevent them from entering the body. A clear statement of a function of mucus.
2. Containing enzymes (e.g., lysozymes) that destroy pathogens. A second clear statement of a function of mucus.

How to earn full marks:

• One mark for each correctly identified function of mucus.
• Accept answers related to preventing pathogens from entering the body.

(b)

1. Some white blood cells (phagocytes) engulf pathogens by phagocytosis. Description of the engulfing process.
2. The pathogen is enclosed in a vesicle/vacuole inside the phagocyte. Further detail of the process.
3. Enzymes digest/break down the pathogen inside the vesicle. Explanation of how the pathogen is destroyed.
4. Other white blood cells (lymphocytes) produce antibodies that bind to specific antigens on pathogens. Description of antibody production.

How to earn full marks:

• One mark for describing phagocytosis.
• One mark for describing the digestion of the pathogen.
• One mark for describing antibody production.
• One mark for mentioning lymphocytes and their role in immunity.

Common Pitfall: Students often mix up the roles of different types of white blood cells. Phagocytes engulf and destroy pathogens, while lymphocytes produce antibodies. Make sure you understand the specific function of each type of cell in the immune response.

Exam-Style Question 3 — Extended Response [8 marks]

Question:

A new strain of antibiotic-resistant bacteria is discovered in a nursing home.

(a) Explain how antibiotic resistance can develop in bacteria populations. [5]

(b) Suggest three measures the nursing home could take to control the spread of this new antibiotic-resistant bacteria. [3]

Worked Solution:

(a)

1. Bacteria reproduce rapidly, leading to many generations in a short time, increasing the chance of mutations. Explains the rapid reproduction rate.
2. Random mutation occurs during DNA replication, creating genetic variation in the bacterial population. Explains the source of variation.
3. Some bacteria may possess a mutation that makes them resistant to a specific antibiotic. Explains the link between mutation and resistance.
4. When antibiotics are used, susceptible bacteria are killed, but resistant bacteria survive and reproduce, passing on the resistance gene. Explains the selective pressure of antibiotics.
5. Over time, the proportion of resistant bacteria in the population increases, leading to a population of antibiotic-resistant bacteria. Explains the increase in resistant bacteria.

How to earn full marks:

• One mark for each point clearly explaining the process of antibiotic resistance development.
• Must include both the role of mutation and the selective pressure of antibiotics.

(b)

1. Isolate infected residents to prevent the spread of the bacteria to other residents and staff. A clear statement of an appropriate measure.
2. Implement strict hygiene protocols, such as frequent and thorough handwashing with antibacterial soap and disinfection of surfaces with appropriate cleaning agents. A second clear statement of an appropriate measure.
3. Review and reduce the use of antibiotics, only prescribing them when absolutely necessary and based on culture and sensitivity testing. A third clear statement of an appropriate measure.

How to earn full marks:

• One mark for each correctly identified measure to control the spread of the antibiotic-resistant bacteria.
• Accept other reasonable answers related to hygiene, isolation, and antibiotic stewardship.

Common Pitfall: Many students forget to mention the role of mutations in creating the initial resistance. Also, be sure to emphasize that it's the proportion of resistant bacteria that increases over time due to the selective pressure of antibiotics. Don't just say "bacteria become resistant."

Exam-Style Question 4 — Extended Response [9 marks]

Question:

A group of students investigated the effectiveness of different types of hand sanitizers in reducing bacterial growth on their hands. They divided into groups, and each group used a different type of hand sanitizer. After using the sanitizer, they pressed their hands onto sterile agar plates. The plates were incubated, and the number of bacterial colonies was counted after 48 hours.

(a) State the type of immunity provided by a vaccination. [1]

(b) Describe the process of vaccination. [4]

(c) Evaluate the experimental design of this investigation and suggest two improvements. [4]

Worked Solution:

(a)

1. Active immunity. Correct identification of the type of immunity.

How to earn full marks:

• One mark for stating "active immunity."

(b)

1. Weakened or inactive pathogens (or antigens) are introduced into the body, usually by injection. Describes the introduction of the pathogen/antigen.
2. The antigens stimulate lymphocytes. Explanation of antigen recognition.
3. Lymphocytes are stimulated to produce specific antibodies that bind to the antigens. Explains antibody production.
4. Memory cells are produced, providing long-term immunity against future infections by the same pathogen. Explains the production of memory cells.

How to earn full marks:

• One mark for each point clearly describing the process of vaccination.
• Must mention the introduction of antigens, antibody production, and memory cell formation.

(c)

1. The experiment is well-designed because it includes a control group ("No Sanitizer"), allowing for comparison. Identifies a positive aspect of the experimental design.
2. The replication (5 students per sanitizer) increases the reliability of the results by reducing the impact of individual variations. Identifies a positive aspect of the experimental design.
3. Improvement 1: Ensure all students wash their hands with the same soap and water before applying the sanitizer. This controls for the initial bacterial load on their hands. Suggests a valid improvement to control a variable.
4. Improvement 2: Use a standardized method for pressing hands onto the agar plates (e.g., same pressure, same duration). This will ensure consistent transfer of bacteria. Suggests a valid improvement to control a variable.

How to earn full marks:

• One mark for identifying a positive aspect of the experimental design.
• One mark for explaining why the aspect is positive.
• One mark for each suggested improvement that is reasonable and addresses a potential weakness in the experimental design.
• Improvements must be different from each other.

Common Pitfall: When describing vaccination, many students forget to mention that memory cells are responsible for long-term immunity. When evaluating experimental designs, focus on identifying variables that were not controlled and suggesting specific ways to control them. Avoid vague suggestions like "be more careful."