🦠 B10 Diseases and Immunity: Comprehensive Study Notes

Welcome to the chapter on Diseases and Immunity! This topic is incredibly important because it explains how we get sick and, more importantly, how our amazing bodies fight back and stay healthy. Think of this as learning the science behind germs, infections, and your personal, built-in superhero defense system!

We will cover what causes disease, how it spreads, and the different ways your body protects itself—from simple physical barriers to complex immune responses.

1. Pathogens and Transmissible Diseases (Core Content)

1.1 What is a Pathogen?

A pathogen is simply a disease-causing organism. These are the "bad guys" that invade your body and try to multiply, causing damage and making you feel unwell.

  • Key Definition: A pathogen is a disease-causing organism.
  • Common examples of pathogens include bacteria, fungi, protoctists, and viruses (we cover viruses in more detail later!).

1.2 Transmissible Diseases

Not all diseases are caused by pathogens (e.g., Type 1 diabetes is not), but many are. If a disease can be spread from one person (or host) to another, it is called a transmissible disease.

  • Key Definition: A transmissible disease is one where the pathogen can be passed from one host to another.

1.3 How Pathogens are Transmitted

Pathogens need a route to travel between hosts. Transmission can be broken down into two main types:

A. Direct Contact

This means the pathogen passes directly between individuals.

  • Examples:
    • Touching, kissing, or sexual contact.
    • Transfer through blood and other body fluids (e.g., shared needles).
B. Indirect Transmission

This means the pathogen uses an intermediary (something in the environment or another organism) to travel.

  • Examples:
    • From contaminated surfaces (like a door handle or phone—these are sometimes called *fomites*).
    • Through contaminated food (food poisoning).
    • Via animals (vectors), such as mosquitoes spreading malaria.
    • Through the air (coughing or sneezing produces droplets that contain pathogens).
Quick Review: Pathogens cause transmissible diseases. Transmission routes are Direct (body fluids, contact) or Indirect (air, food, surfaces, animals).

2. The Body's Defenses Against Pathogens (Core Content)

Your body has incredible mechanisms to stop pathogens entering and, if they do get inside, to destroy them. Think of your body as a high-security castle!

2.1 The First Line of Defense (Physical Barriers)

These barriers try to stop pathogens from ever entering the body.

  1. Skin: The largest organ. It provides a tough, waterproof, physical barrier that is difficult for most pathogens to penetrate unless it is broken (e.g., by a cut). (Analogy: The castle walls.)
  2. Hairs in the nose: These trap dust and microbes entering the respiratory system during inhalation.
  3. Mucus: A sticky secretion found in the nose and airways (trachea, bronchi). It traps inhaled pathogens before they reach the lungs.
  4. Stomach Acid: The hydrochloric acid in the stomach is highly acidic (low pH). It kills most harmful microorganisms found in food or drink that are ingested. (Analogy: A deadly acid trap in the moat!)

2.2 The Second Line of Defense (White Blood Cells)

If pathogens successfully enter the body, the internal immune system takes over, led by white blood cells.

White blood cells have two primary roles (as listed in B9.4 Blood):

  1. Phagocytosis: Some white blood cells surround, engulf, and digest the pathogens. This is a non-specific defense—they eat anything foreign. (Think of Pac-Man cells eating germs.)
  2. Antibody Production: Other white blood cells (lymphocytes) produce specific chemicals called antibodies to destroy the pathogen. This is a specific defense, as each antibody is shaped to attack one specific type of pathogen.
Key Takeaway: Defenses include physical barriers (skin, mucus, hairs, stomach acid) and internal cellular defense (white blood cells using phagocytosis and antibody production).

3. Viruses: A Special Type of Pathogen (Supplement Content)

Viruses are unique and often tricky pathogens because they are not technically cells and rely completely on a host cell to reproduce.

Features of Viruses (B10.1 Supplement 5)

Compared to bacteria or fungi, the features of a virus are very simple, limited to:

  1. A protein coat (capsid) surrounding the internal material.
  2. Genetic material (DNA or RNA) inside the coat.

Did you know? Because viruses lack a cell structure and metabolism, they are not affected by antibiotics. Antibiotics only kill bacteria. This is why you cannot treat a common cold (caused by a virus) with antibiotics!

4. Controlling the Spread of Disease (Supplement Content)

We can significantly reduce the incidence of transmissible diseases through effective public health measures.

Importance of Control Measures (B10.1 Supplement 6)

Controlling the spread of pathogens involves breaking the chain of transmission:

  1. A clean water supply: This prevents the spread of waterborne diseases (like cholera), ensuring water is free from pathogens (often achieved through purification and chlorination).
  2. Hygienic food preparation: Prevents pathogens (especially bacteria like Salmonella) from contaminating and multiplying in food. This includes keeping food cool and cooking it thoroughly.
  3. Good personal hygiene: Simple actions like washing hands regularly, covering coughs/sneezes, and showering prevent pathogens from spreading by direct contact or air.
  4. Waste disposal: Properly disposing of solid waste (rubbish) prevents breeding grounds for pests (like rats and flies) that can act as vectors for disease.
  5. Sewage treatment: Treating human waste removes pathogens before the water is released back into the environment, preventing widespread contamination of water sources. (No need to know the detailed stages of this process.)
Memory Aid: Think of the 5 Clean Ups: Clean Water, Clean Food, Clean Hands, Clean Waste, Clean Sewage.

5. Active Immunity (Supplement Content)

The best long-term defense against a pathogen is to have immunity—a state where your body can quickly recognize and destroy the pathogen before it causes symptoms.

5.1 Describing Active Immunity (B10.1 Supplement 7)

Active immunity is the defense against a pathogen that is achieved by the body making its own antibodies.

  • When your body encounters a pathogen (an organism carrying specific antigens—markers on its surface), your white blood cells (lymphocytes) start a process to produce specific antibodies that match and neutralize those antigens.
  • The primary exposure is slow, but once successful, the body creates memory cells.
  • If the body encounters the same pathogen again, the memory cells immediately trigger a fast, large-scale production of antibodies, preventing the disease from developing.

5.2 Gaining Active Immunity (B10.1 Supplement 8)

Active immunity can be gained in two ways:

  1. After an infection by a pathogen: This is natural active immunity. Once you recover from chickenpox, your body has memory cells that keep you safe from getting it again.
  2. By vaccination: This is artificial active immunity.

Vaccination:

  • A vaccine contains a dead, weakened, or inactive form of the pathogen (or just its antigens).
  • This is injected into the body.
  • The body is tricked into thinking it is under attack and produces antibodies and memory cells without you experiencing the actual disease.
  • If the real, live pathogen enters the body later, the memory cells are ready and provide immediate protection.
Key Takeaway: Active immunity is gained naturally (after infection) or by vaccination. Both result in the body's white blood cells producing antibodies and long-lasting memory cells.