Sciences - Co-ordinated (Double) (0654) | Diseases and immunity

B10: Diseases and Immunity – Comprehensive Study Notes

Hello future biologists! This chapter is incredibly important, not just for your exams, but because it explains how your body fights off infection and how we manage diseases globally. Don't worry if some of the terms seem new; we’ll break down how pathogens attack and how your brilliant immune system defends you!

1. Pathogens and Transmissible Diseases (Core & Supplement)

What Causes Disease?

A disease-causing organism is called a pathogen. Pathogens are usually tiny, microscopic living things (or borderline living things, like viruses).

The main types of pathogens include:

• Bacteria (e.g., Salmonella causing food poisoning)
• Fungi (e.g., athlete's foot)
• Protozoa (e.g., Plasmodium causing malaria)
• Viruses (e.g., Flu, HIV)

Did you know? (Supplement)

Viruses are unique and usually consist only of a protein coat surrounding their genetic material (DNA or RNA). They are not technically classified as cells and must hijack a host cell to reproduce. This is why antibiotics don't work against them!

Transmissible Diseases

A transmissible disease is one where the pathogen can be passed from one living host to another.

How are Pathogens Transmitted? (Core)

Transmission can happen in many ways. You need to know both direct and indirect routes:

A. Direct Contact (The Close Route)

• Direct physical contact (shaking hands, kissing).
• Contact through blood and other body fluids (e.g., sexual contact, shared needles).

B. Indirect Contact (The Environment Route)

• From contaminated surfaces or objects (fomites). Example: Touching a door handle used by an infected person.
• Through food (e.g., unwashed produce, contaminated meat).
• Via animals (vectors), such as mosquitoes (malaria) or flies.
• Through the air (coughing/sneezing releases droplets).

2. The Body’s Defenses Against Pathogens (Core)

Your body has several natural defense mechanisms designed to stop pathogens from ever getting into your bloodstream or vital organs. These are mostly non-specific barriers, meaning they target all intruders equally.

1. Physical Barriers:

• Skin: An unbroken physical barrier covering the body, preventing entry.
• Hairs in the nose: Trap dust and microbes from inhaled air.
• Mucus: Sticky substance secreted in the respiratory system (trachea and bronchi) that traps pathogens.

2. Chemical Barriers:

• Stomach Acid: Hydrochloric acid in the stomach kills most microorganisms found in ingested food and drink, due to the very low (acidic) pH.

3. Cellular Defense:

• White Blood Cells (WBCs): These are the immune system's army. They constantly patrol the body in the blood and lymph.

3. The Immune System: Specific Defense (Core & Supplement)

When a pathogen breaks through the initial barriers, the immune system launches a highly specific counter-attack. This is primarily done by specialized White Blood Cells (WBCs), specifically the lymphocytes and phagocytes.

Key Players in Specific Immunity

1. Phagocytes (Core & Supplement)
These WBCs are like the ‘pac-men’ of the body. Their job is phagocytosis: engulfing and destroying pathogens by absorbing them and breaking them down using enzymes. They are non-specific but critical for defense.

2. Lymphocytes (Supplement)
These are the WBCs responsible for targeted immunity, primarily through the production of antibodies.

3. Antigens (Supplement)
Pathogens have unique molecules, usually proteins, on their surface called antigens. Think of an antigen as the pathogen's "ID badge" or "uniform". Each type of pathogen has its own specific antigen shape.

4. Antibodies (Supplement)
These are specialized proteins produced by lymphocytes in response to a specific antigen.

• Antibodies have a specific, complementary shape that allows them to fit and bind only to a specific type of antigen.
• When bound, antibodies either directly destroy the pathogen or, more often, mark the pathogens so that phagocytes can easily find and destroy them.

Analogy: If an antigen is a key, the antibody is the specific lock that fits perfectly.

Active Immunity (Supplement)

Active immunity is the defense against a pathogen achieved by the body itself producing antibodies. This immunity lasts a long time because the body remembers how to make the antibodies.

It is gained in two main ways:

1. After an infection by a pathogen (Natural Active Immunity).
2. By vaccination (Artificial Active Immunity).

4. Controlling the Spread of Disease (Core)

Controlling transmissible diseases relies heavily on good public health practices to break the chains of transmission.

Importance of Public Health Measures:

• A Clean Water Supply: Prevents the spread of water-borne diseases like cholera, which are often caused by faecal contamination.
• Hygienic Food Preparation: Reduces the risk of food poisoning by avoiding contamination and ensuring food is cooked properly to kill pathogens.
• Good Personal Hygiene: Simple actions like regular hand washing (especially after using the toilet or before eating) drastically reduce direct and indirect transmission.
• Waste Disposal: Proper collection and safe disposal of rubbish removes breeding grounds for pests (like rats and insects) that can act as disease vectors.
• Sewage Treatment: Safely processing human waste prevents pathogens from entering the public water supply or environment. (Note: You do not need to recall the specific stages of sewage treatment.)

5. Vaccination: Artificial Active Immunity (Core & Supplement)

Vaccination is one of the most successful methods of controlling transmissible diseases globally.

The Role of Vaccination (Core & Supplement)

Vaccinations are available for some pathogens and help control the spread of diseases by achieving herd immunity—when enough of the population is immune, the pathogen cannot easily find new hosts.

The Process of Vaccination (Supplement)

A vaccine works by safely exposing your immune system to the pathogen's identity without causing the disease itself.

Step-by-Step Explanation:

1. Input: A vaccine contains weakened (attenuated) pathogens or just their antigens (protein coats) which are put into the body. These are harmless but look real to the immune system.
2. Stimulation: The antigens stimulate an immune response by specific WBCs called lymphocytes.
3. Production: These lymphocytes multiply and produce large quantities of antibodies specific to the introduced antigens.
4. Memory: Crucially, some lymphocytes develop into memory cells.

The memory cells remain in the blood for years, providing long-term immunity. If the real, strong pathogen ever enters the body, the memory cells quickly recognize the antigens and produce massive amounts of antibodies immediately, destroying the pathogen before it can cause symptoms.

Analogy: Vaccination is like attending a fire drill. You learn how to respond to a real fire (infection) safely, so you are ready immediately if the alarm ever sounds.