Biology | Defence against disease

🔬 Defence Against Disease: Your Body's Personal Security System

Hello future biologists! Welcome to one of the most fascinating topics in the "Interaction and Interdependance" section: how your body fights off intruders. This chapter is all about understanding the incredible ways your biological systems interact to keep you healthy, acting as a complex defense network.

Don't worry if concepts like "B cells" or "T cells" seem confusing right now. We'll break down this amazing security system into three easy-to-understand lines of defense. By the end, you'll know exactly why you don't get sick from the same cold twice!

Key Terms to Master

• Pathogen: An organism or virus that causes disease (e.g., bacteria, viruses, fungi, protozoa).
• Antigen: A molecule (usually protein or polysaccharide) found on the surface of a pathogen that triggers an immune response.
• Antibody: A protein produced by plasma cells in response to a specific antigen; its role is to neutralize or mark the antigen for destruction.

1. The First Line of Defence: Keeping the Doors Locked

The first line of defense is non-specific, meaning it doesn't care what the pathogen is—it just blocks everything. Think of this as your body's physical fortress walls and moats.

The Barriers

A. Skin (The Physical Wall)

Your skin is the primary barrier against pathogens.

• Structure: The outer layer (epidermis) consists of tough, dead cells that are difficult for pathogens to penetrate.
• Protection: It secretes oils and fatty acids, making the surface slightly acidic (\( \text{pH} \approx 5 \)), which inhibits the growth of many bacteria.
• Healing: When the skin is cut, clotting quickly seals the breach, preventing pathogens from entering the bloodstream.

B. Mucous Membranes (The Sticky Traps)

These membranes line the internal tubes of the body (respiratory tract, digestive tract, urinary tract).

• Mucus: They secrete a sticky substance called mucus, which traps pathogens that are inhaled or swallowed.
• Cilia: In the respiratory system, tiny hair-like structures called cilia sweep the mucus (and trapped pathogens) away to be swallowed or expelled (e.g., sneezing or coughing).
• Chemical Defences: Some membranes, like those in tears or saliva, produce lysozyme, an enzyme that chemically breaks down bacterial cell walls.

2. The Second Line of Defence: The Internal Security Guards

If a pathogen manages to breach the first line (e.g., via a cut), the second line activates. This defense is still non-specific but is internal and involves various types of white blood cells (leukocytes).

Phagocytic White Blood Cells (The Pac-Men)

The most important cells in this phase are the phagocytes (like macrophages and neutrophils). Phagocytes are specialized white blood cells that engulf and destroy foreign material.

Analogy: Imagine phagocytes are the internal security guards patrolling the hallways. If they spot any debris or an unfamiliar face, they immediately surround and consume it.

Step-by-Step: The Process of Phagocytosis

1. Detection: Phagocytes move to the site of infection (often signaled by chemical messengers).
2. Engulfment: The phagocyte extends its membrane (pseudopods) around the pathogen.
3. Vesicle Formation: The pathogen is enclosed in an internal sac called a phagosome.
4. Digestion: The phagosome fuses with a lysosome (which contains powerful digestive enzymes).
5. Elimination: The enzymes break down the pathogen. Undigested debris is expelled from the cell.

Inflammation

Phagocytosis often occurs during the process of inflammation. Inflammation is a localized tissue response characterized by redness, swelling, heat, and pain. This happens because damaged cells release chemicals (like histamine) that increase blood flow to the area, bringing more phagocytes to the fight.

3. The Third Line of Defence: Specific Immunity (The Specialists)

If the infection persists, the adaptive immune system takes over. This system is powerful because it can "remember" past invaders and launch a faster, stronger response upon re-exposure. This line relies on two types of specialized white blood cells: lymphocytes (B cells and T cells).

Antigens and the Lock-and-Key Principle

The immune system recognizes pathogens by their antigens. An antigen is like a specific molecular "name tag" on the surface of the intruder.

• Each pathogen has unique antigens.
• Each B lymphocyte has a unique surface receptor (antibody) that is specific to only one antigen.

Antibodies: The Custom-Made Weapons

Antibodies are globular proteins (immunoglobulins) shaped like a "Y".

• They have a variable region at the tips of the 'Y' arms, which is highly specific and binds precisely to one type of antigen (the lock-and-key mechanism).
• Their main functions are:
  • Neutralization: Binding to toxins or viruses, preventing them from infecting host cells.
  • Opsonization: Coating the pathogen, making it easier for phagocytes to engulf it.
  • Agglutination: Clumping pathogens together, making them easier targets for phagocytes.

The Process of Specific Immunity: Clonal Selection

This process explains how the immune system rapidly produces massive amounts of the right antibody when needed.

Step 1: Antigen Presentation and Recognition

When a phagocyte destroys a pathogen, it often displays the pathogen's antigens on its own surface (becoming an Antigen-Presenting Cell). A B-lymphocyte with the matching receptor (antibody) recognizes and binds to this antigen.

Step 2: Clonal Selection and Activation

Once the specific B cell is activated (often requiring help from T cells), it undergoes clonal selection. This means it rapidly divides by mitosis to form a large population (clone) of genetically identical cells.

Step 3: Differentiation

The cloned cells differentiate into two main types:

1. Plasma Cells: These are short-lived, antibody factories. They produce and secrete vast quantities of the specific antibody required to fight the current infection.
2. Memory Cells: These are long-lived cells that remain in the body. They don't actively fight the infection now, but they ensure that the body is ready to mount a rapid defense if the same pathogen attacks again.

4. Immunity: Primary, Secondary, and Vaccination

Primary vs. Secondary Immune Response

The difference between the first time you encounter a pathogen and the second time is crucial to understanding immunity.

• Primary Response: The first time you encounter an antigen. It is slow (3–7 days needed for B cells to multiply) and produces a relatively low concentration of antibodies. You usually feel sick during this phase.
• Secondary Response: The second or subsequent time you encounter the same antigen. The memory cells quickly recognize the pathogen, resulting in a much faster, stronger, and more sustained antibody production. This response is often so rapid that you never show symptoms (you are immune).

The Principle of Vaccination

Vaccination is the deliberate introduction of an antigen to produce immunity.

• Mechanism: Vaccines contain weakened, inactive, or fragmented parts of a pathogen (antigens) that are non-pathogenic (cannot cause the full disease).
• Stimulation: When injected, these antigens stimulate a primary immune response, leading to the creation of B and T memory cells.
• Protection: If the real, active pathogen enters the body later, the memory cells immediately trigger the rapid secondary response, providing effective immunity without the host ever having suffered the disease.

Concept Link: Vaccination provides artificial active immunity. "Active" because the person's own immune system made the antibodies and memory cells.

5. Application: Immune System Failure (HL Focus/Deep Dive)

While the general mechanism of immunity is covered in SL, understanding how these mechanisms can be compromised is essential.

HIV and AIDS

Human Immunodeficiency Virus (HIV) is a serious example of how a pathogen can directly target and cripple the immune system.

• Target: HIV specifically attacks and destroys a type of lymphocyte called helper T cells. Helper T cells are vital because they coordinate the specific immune response (they help activate B cells and other T cells).
• Progression: As the helper T cell count drops, the immune system becomes progressively weaker.
• AIDS: When the helper T cell count falls below a critical level, the person is diagnosed with Acquired Immune Deficiency Syndrome (AIDS). At this stage, the body cannot fight off opportunistic infections (diseases that a healthy immune system could easily handle).

Key Takeaway for Immune Deficiencies: The immune system is an interdependent network. Removing one critical component (like helper T cells) causes the entire adaptive response to collapse.