Biology (9700) | The immune system

The Immune System (Topic 11): Your Body's Ultimate Defence Force

Welcome to one of the most exciting and complex topics in Biology: The Immune System!
This chapter is all about how your body defends itself against microscopic invaders like bacteria, viruses, and fungi. Understanding immunity is vital for grasping concepts like infectious diseases, vaccination, and modern medical treatments.
Don't worry if the names of the cells seem confusing at first—we'll break down this complex system into two clear defensive lines: the quick, non-specific response, and the highly trained, specific response. Let's get started!

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11.1 The Immune System: Non-Specific Defence (The First Responders)

The first line of defence against pathogens is often referred to as the non-specific or innate immune system. This system acts immediately, treating all foreign threats the same way. The main cells involved here are the phagocytes.

Mode of Action of Phagocytes

Phagocytes act like the clean-up crew of your body. Their job is to literally ‘eat’ pathogens. The two main types you need to know are Macrophages and Neutrophils.

Step-by-step: How Phagocytes Work (Phagocytosis)

1. Chemotaxis: Phagocytes are attracted to the infection site by chemical signals released by damaged cells or the pathogens themselves.
2. Adhesion: The phagocyte recognises and attaches to the pathogen (often facilitated by surface proteins).
3. Engulfment: The phagocyte extends its cytoplasm to surround the pathogen, enclosing it within a vesicle called a phagosome.
4. Digestion: The phagosome fuses with a lysosome (which contains powerful digestive enzymes like lysozyme). The resulting structure is called a phagolysosome.
5. Elimination: The enzymes break down the pathogen. The harmless breakdown products are absorbed, and waste material is expelled by exocytosis.

Quick Analogy: Imagine a Pac-Man (the phagocyte) chasing dots (the pathogens), swallowing them (phagosome), and breaking them down inside its stomach (lysosome).

Antigens: Identity Tags

For the specific immune system to work, it needs to know who the enemy is. This identification is based on antigens.

An Antigen is any molecule, usually a protein or glycoprotein (see Topic 4.1.3), found on the surface of a cell that can trigger an immune response.

1. Self Antigens: These are antigens found on the surface of your own body cells. The immune system is normally trained to ignore these.
2. Non-Self Antigens: These are foreign antigens found on pathogens, transplanted tissues, or cancerous cells. These trigger the immune response.

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11.1 The Immune System: Specific Defence (The Adaptive Response)

When phagocytosis isn't enough, the specific or adaptive immune system kicks in. This response is much slower initially but highly effective because it targets a *specific* antigen. The key cells here are the Lymphocytes, which are divided into two main groups:

• B-lymphocytes (B cells) – Responsible for humoral immunity (antibody production).
• T-lymphocytes (T cells) – Responsible for cell-mediated immunity (destroying infected cells).

The Primary Immune Response Sequence

This is the first time the body encounters a specific pathogen. The sequence of events is complex, but you can think of it as a coordinated military operation:

Phase 1: Detection and Presentation

1. Macrophage Engulfs Pathogen: A macrophage performs phagocytosis on the pathogen.
2. Antigen Presentation: Instead of simply discarding the remains, the macrophage takes the pathogen’s specific antigens and displays them on its own cell surface membrane. The macrophage is now an Antigen Presenting Cell (APC).

Phase 2: Activation and Clonal Selection

3. T-Helper Cell Activation: A specific T-helper cell (T_H cell) that has a receptor complementary to the presented antigen binds to the APC. This binding activates the T-helper cell.
4. Clonal Expansion of T-Helper Cells: The activated T-helper cell rapidly divides (mitosis) to produce a large clone of identical T-helper cells.
5. B-lymphocyte Activation (Clonal Selection): Meanwhile, specific B-lymphocytes, which have surface antibodies complementary to the antigen, bind directly to the pathogen (or are stimulated by the activated T-helper cells). This selection process is called clonal selection.

Phase 3: Action and Elimination

6. B-cell Differentiation: The selected B-lymphocytes divide rapidly (clonal expansion) and differentiate into two cell types:

• Plasma Cells: These are antibody factories. They produce and secrete large amounts of specific antibodies (proteins) into the blood and tissue fluid.
• Memory B Cells: These provide long-term protection (see next section).

7. T-Killer Cell Activation: T-helper cells also activate T-killer cells (T_K cells). These T-killer cells specifically seek out and destroy body cells that have been infected by the pathogen (e.g., virus-infected cells) by releasing toxins.
8. Pathogen Clearance: Antibodies bind to the antigens, neutralising the pathogen or marking it for destruction by phagocytes. The infection is cleared.

Memory Aid: Remember the T-cells as the "managers" and B-cells as the "bomb makers."

Secondary Immune Response and Long-Term Immunity

The primary response takes time (often days) because the correct B-cell needs to be found, selected, and cloned. However, if the same pathogen attacks again, the response is dramatically faster and stronger.

Role of Memory Cells: During the primary response, some B-lymphocytes and T-lymphocytes differentiate into Memory Cells.
When the body encounters the same non-self antigen a second time:

• The memory cells immediately recognise the antigen.
• They undergo rapid clonal expansion and differentiation (much faster than in the primary response).
• They produce antibodies much faster and in much higher concentrations.

This rapid and overwhelming response eliminates the pathogen before symptoms of the disease appear, providing long-term immunity.

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11.2 Antibodies and Vaccination

1. The Structure and Function of Antibodies

Antibodies are Y-shaped, large, soluble proteins (globular proteins) called immunoglobulins.

Molecular Structure:

• Composed of four polypeptide chains: two long heavy chains and two short light chains.
• These chains are held together by disulfide bonds (a type of covalent bond).
• The structure has a constant region (C) and a variable region (V).
• The variable region, located at the ends of the 'Y' arms, forms the antigen-binding site. The shape of this site is highly specific and complementary to a particular antigen.

Function (Relating Structure to Function):
Since the antibody is bivalent (it has two binding sites), it can bind to two different antigens simultaneously, leading to several outcomes:
1. Agglutination (Clumping): Antibodies bind multiple pathogens together, forming large clumps. These clumps are too large to infect cells and are easily engulfed and destroyed by phagocytes.
2. Neutralisation: Antibodies bind directly to the toxic sites on pathogens or toxins, blocking them from entering or damaging host cells.

2. Monoclonal Antibodies (MABS)

Sometimes, we need large quantities of a single, highly specific type of antibody outside the body for medical use. These are called monoclonal antibodies.

Outline of the Hybridoma Method

Monoclonal antibodies are produced using the hybridoma method. This technique fuses an antibody-producing cell with a cancerous cell:
1. A mouse (or other animal) is injected with the desired antigen to stimulate a specific primary immune response, causing B-lymphocytes to produce antibodies.
2. These specific B-lymphocytes are harvested from the animal's spleen.
3. These B-cells are fused with myeloma cells (cancerous plasma cells), which can divide indefinitely.
4. The resulting fused cell is called a hybridoma. This hybridoma cell has two important features: it produces the desired specific antibody (from the B-cell) and it can replicate endlessly (from the myeloma cell).
5. These hybridoma cells are cultured (grown) in vast quantities to produce large amounts of identical monoclonal antibodies.

Principles of Using Monoclonal Antibodies

MABS are incredibly useful due to their specificity:
1. Diagnosis of Disease:
Example: Pregnancy Testing. Monoclonal antibodies are used to bind to the hormone human chorionic gonadotropin (hCG) found in the urine of pregnant women. When the antibody-hCG complex forms, a colour change is triggered.
2. Treatment of Disease:
MABS can be attached to drugs or radioactive substances. Since the antibody is specific to a target antigen (e.g., on a cancer cell), the drug is delivered directly to the required site, minimising damage to healthy cells.

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11.2 Immunity Types and Vaccination

3. Active vs. Passive Immunity

Immunity can be classified based on whether the body *makes* the antibodies (Active) or *receives* them ready-made (Passive).

Active Immunity:

• The body is stimulated to produce its own antibodies and memory cells.
• It takes time to develop (primary response).
• It provides long-term immunity.

Passive Immunity:

• The individual receives ready-made antibodies from an external source.
• Protection is immediate.
• No memory cells are formed, so protection is short-term (the antibodies are eventually broken down).

4. Natural vs. Artificial Immunity

These two categories combine to define how the immunity was acquired.

A. Natural Immunity: Occurs without deliberate medical intervention.

• Natural Active Immunity: Developing immunity after suffering an infection (e.g., recovering from the flu).
• Natural Passive Immunity: Antibodies passing from mother to baby (across the placenta or in breast milk).

B. Artificial Immunity: Acquired through medical procedures.

• Artificial Active Immunity: Immunity resulting from vaccination (deliberate introduction of antigens).
• Artificial Passive Immunity: Receiving an injection of ready-made antibodies (e.g., antivenom for a snake bite).

5. The Role of Vaccination

A vaccine contains antigens (often attenuated, dead, or fragments of pathogens) that are harmless but sufficient to stimulate an immune response.

How Vaccines Provide Long-Term Immunity:
1. The vaccine (antigen) is injected into the body.
2. The body mounts a primary immune response against the antigen.
3. The pathogen is cleared, and crucially, memory cells (both B and T) are produced.
4. If the real, active pathogen enters the body later, the memory cells trigger a rapid, overwhelming secondary immune response, providing effective long-term protection.

6. Vaccination Programmes and Disease Control

Vaccination programmes are essential for controlling the spread of infectious diseases across a population.
The key concept here is Herd Immunity.

Herd Immunity: When a sufficiently large proportion of the population is vaccinated (typically 80–95%), the spread of the pathogen is drastically reduced. This indirectly protects individuals who cannot be vaccinated (e.g., infants, or those with compromised immune systems), as the probability of the pathogen meeting a susceptible host drops dramatically.

Did you know? Global vaccination programmes, such as for smallpox, have led to the complete eradication of that disease worldwide, demonstrating the profound impact of artificial active immunity.