Biology (0610) | Diseases and immunity

🔬 IGCSE Biology Study Notes: Diseases and Immunity (0610)

Welcome to the fascinating world of Diseases and Immunity! This chapter explains how your body fights off microscopic invaders and how public health keeps us safe. Understanding these biological battles is crucial for controlling global health issues. Don't worry if some of the terms seem complex—we'll break them down step-by-step!

1. Understanding Disease and Transmission (Core Content)

1.1 Pathogens and Transmissible Diseases

A pathogen is simply any disease-causing organism. These tiny invaders include bacteria, viruses, fungi, and protoctists.

• A transmissible disease (or infectious disease) is one where the pathogen can be passed from one host (a person or animal) to another.
• Example: Influenza (Flu) is transmissible; diabetes is not.

1.2 How Pathogens Spread (Transmission)

Pathogens need a way to travel from a sick host to a healthy one. This travel route is called transmission.

A. Direct Contact

This involves physical contact between hosts or contact with direct bodily substances.

• Through blood and other body fluids (e.g., sharing contaminated needles or sexual contact).
• Through direct physical contact (e.g., touching a wound).

B. Indirect Contact

This involves the pathogen travelling via an intermediate substance or organism.

• Contaminated surfaces (Fomites): Touching a doorknob or desk recently touched by an infected person.
• Food and water: Consuming contaminated food or water (e.g., cholera transmitted via dirty water).
• Animals (Vectors): An organism (like a mosquito) that carries the pathogen but is not harmed by it (e.g., malaria spread by mosquitoes).
• Air (Aerosols): Droplets released when an infected person coughs or sneezes, which are then inhaled.

2. The Body's Natural Defences (Core Content)

Your body has several lines of defence to stop pathogens from entering and causing harm.

2.1 Physical and Chemical Barriers (First Line of Defence)

These defences act as walls and traps to prevent entry:

• Skin: An intact, tough, waterproof barrier covering the outside of the body.
• Hairs in the nose: Trap dust and pathogens breathed in with the air.
• Mucus: A sticky substance produced in the breathing system (trachea and bronchi) that traps microbes.
• Stomach Acid: Hydrochloric acid creates a low pH environment, killing most harmful microorganisms swallowed with food.

2.2 White Blood Cells (WBCs) (Second Line of Defence)

Once pathogens get past the barriers, WBCs are the mobile defence force in the blood.

• Phagocytes: These cells patrol the blood and tissues. They perform phagocytosis—the process of engulfing ("eating") pathogens and digesting them using enzymes. (Think of Pac-Man consuming dots.)
• Lymphocytes: These cells produce antibodies (which we discuss in detail below).

3. Controlling the Spread of Disease (Core Content)

Public health measures are vital for preventing outbreaks in populations.

It is important to control the spread of disease through:

• A clean water supply: Ensures water is free from pathogens (like the cholera bacterium).
• Hygienic food preparation: Avoiding contamination, cooking food properly to kill bacteria, and washing hands.
• Good personal hygiene: Washing hands regularly, covering coughs/sneezes, and bathing.
• Waste disposal: Properly disposing of rubbish to prevent breeding grounds for vectors (e.g., rats, flies).
• Sewage treatment: Treating human waste before releasing it back into the environment to kill waterborne pathogens. (Note: You only need to know that sewage treatment is important, not the specific stages.)

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4. Advanced Immunity: Antigens and Active Defence (Extended Content)

4.1 Antigens and Antibodies

The immune system is highly specific, meaning it targets one type of pathogen only.

• Antigens: Unique protein markers found on the surface of pathogens (or any foreign substance). Each pathogen has its own specific shape of antigen. (Think of an antigen as a unique ID badge.)
• Antibodies: Proteins produced by lymphocytes. Antibodies have specific, complementary shapes that fit perfectly onto a specific antigen.

How Antibodies Work:

When an antibody binds to a pathogen's antigen, one of two things happens:

1. Direct Destruction: The antibody binding leads directly to the breakdown of the pathogen.
2. Marking for Destruction: The antibody 'tags' or 'marks' the pathogen, making it easier for phagocytes to find and engulf it.

4.2 Active Immunity

Active immunity is the defence against a pathogen achieved by your own body producing antibodies. This type of immunity is long-term because it involves creating "memory."

Active immunity can be gained in two ways:

1. After infection: When you naturally catch a disease, your lymphocytes produce antibodies and special memory cells. If you meet the pathogen again, the memory cells react instantly and powerfully.
2. By vaccination: This is controlled, artificial exposure.

4.3 The Process of Vaccination (Immunisation)

Vaccination is a crucial way to control the spread of disease.

1. Introduction: Weakened, dead pathogens, or just their antigens (the surface proteins), are put into the body (usually by injection).
2. Stimulation: The antigens stimulate the lymphocytes to launch an immune response and produce specific antibodies. Since the pathogen is weakened, you don't actually get sick, or you only get mild symptoms.
3. Memory: Crucially, memory cells are produced. These cells remain in the blood for a long time.
4. Protection: If the real, strong pathogen enters the body later, the memory cells immediately trigger a fast, powerful, antibody response. This gives long-term immunity.

Role of Vaccination in Controlling Spread:
Vaccination protects individuals, but when a large portion of the population is vaccinated, it creates herd immunity. This means the disease cannot spread easily, protecting vulnerable individuals who cannot be vaccinated (like babies or those with weak immune systems).

5. Passive Immunity (Extended Content)

Passive immunity is a short-term defence against a pathogen achieved by receiving antibodies made by *another* individual.

• Short-Term Defence: The protection only lasts as long as the acquired antibodies stay in the bloodstream.
• Crucial Point: Memory cells are NOT produced in passive immunity. Your body did not do the work to make the antibodies itself.

Sources of Passive Immunity:

1. Across the placenta: Antibodies can pass from the mother's blood to the fetus's blood during pregnancy, providing the baby with protection for the first few months of life.
2. In breast milk: Antibodies found in breast milk are passed to the infant through feeding. This is vital for the development of passive immunity in infants.

6. Case Study: Cholera (Extended Content)

6.1 Cause and Transmission

• Disease: Cholera
• Cause: A bacterium, Vibrio cholerae.
• Transmission: Ingested through contaminated water supplies (often due to poor sanitation and sewage mixing with drinking water).

6.2 Mechanism of Action

The cholera bacterium causes severe, life-threatening diarrhoea through the action of a toxin it produces.

The steps of the disease mechanism are:

1. The cholera bacterium enters the small intestine.
2. It produces a toxin.
3. This toxin stimulates the cells lining the small intestine to actively secrete large amounts of chloride ions (\(Cl^-\)).
4. The increased concentration of ions in the gut creates a low water potential in the gut lumen.
5. Water then moves out of the body tissues and into the gut by osmosis.
6. This massive movement of water results in severe diarrhoea, leading to rapid dehydration and loss of essential ions from the blood.