Defence Mechanisms (AQA A-Level Biology): Revision Notes
Defence Mechanisms
Millions of people die from infectious diseases each year, yet many others survive infections or show resistance to pathogens. Understanding how our body's defence systems work helps explain these differences in susceptibility and recovery.
The human body's defence mechanisms operate as an integrated system where multiple layers of protection work together. The outcome of any infection depends on the complex interaction between pathogen strength and the body's defensive capabilities.
Understanding infection and immunity
An infection represents an interaction between a pathogen (disease-causing organism) and the body's defence systems. The outcome depends on which side gains the upper hand. When the body successfully fights off an infection, it often develops immunity - a state where the defence mechanisms are better prepared to combat the same pathogen in future encounters.
The effectiveness of our defences varies significantly between individuals. Healthy adults rarely die from infections, while those in poor health, young children, and elderly people face greater vulnerability due to less robust immune responses.
Factors Affecting Immune Response Strength:
Individuals with compromised immune systems face significantly higher risks from infections that healthy people easily overcome. This includes people with chronic illnesses, those taking immunosuppressive medications, and individuals at the extremes of age (very young or elderly).
Types of defence mechanisms
The human body employs multiple defence strategies that work together to protect against pathogens. These can be categorised into two main types based on their speed and specificity:
Non-specific defences
These provide immediate protection and respond identically to all pathogens. Examples include:
- Physical barriers such as skin
- Phagocytosis - the process where specialised cells engulf and destroy foreign material
Specific defences
These responses develop more slowly but target particular pathogens with precision. They involve lymphocytes (a type of white blood cell) and take two forms:
- Cell-mediated responses involving T lymphocytes
- Humoral responses involving B lymphocytes
The two-tier defence system provides both immediate protection through non-specific mechanisms and long-term, targeted protection through specific responses. This dual approach ensures the body can respond quickly to threats while building stronger, more targeted defences over time.
Recognising self from non-self
For defence mechanisms to work effectively, the body must distinguish between its own cells (self) and foreign material (non-self). Without this ability, lymphocytes would attack the body's own tissues, causing severe damage.
The role of surface proteins
Each cell type carries specific protein molecules on its surface that act as molecular identification tags. These proteins have complex three-dimensional structures that make each cell type unique. The immune system uses these protein markers to identify:
- Pathogens - such as viruses, bacteria, and fungi
- Non-self material - including cells from other organisms of the same species
- Toxins - harmful substances produced by certain pathogens
- Abnormal cells - such as cancer cells
Medical Implications of Self vs Non-Self Recognition:
This recognition system has critical implications for medical procedures like organ transplants. The immune system identifies donated organs as non-self and attempts to reject them, which is why transplant patients must take immunosuppressive drugs to prevent rejection.
How lymphocytes develop recognition abilities
The body produces approximately ten million different types of lymphocytes, each capable of recognising different molecular shapes. This diverse army of cells undergoes a careful selection process:
Early development
- Lymphocytes initially encounter various materials during development
- In the foetus, they primarily contact the body's own cells and proteins
- Those lymphocytes with receptors matching self-antigens either die or become suppressed
- Only lymphocytes that might respond to foreign (non-self) material survive
The selection advantage
This selection process ensures that:
- No lymphocytes remain that would attack the body's own tissues
- Only lymphocytes capable of recognising foreign threats persist
- The immune system maintains its protective function without causing self-harm
Example: Lymphocyte Development Process
Step 1: The body produces millions of different lymphocyte types during development Step 2: These lymphocytes encounter the body's own proteins and cells in the foetus Step 3: Any lymphocytes that recognise "self" materials are eliminated or suppressed Step 4: Only lymphocytes that could potentially respond to "non-self" materials survive Result: A mature immune system that protects against foreign threats without attacking the body's own tissues
Clonal selection and response timing
When a pathogen enters the body, specific lymphocytes with complementary surface receptors recognise it. These selected lymphocytes then undergo clonal selection - rapidly dividing to build up their numbers to levels effective for destroying the pathogen.
This multiplication process explains the time lag between pathogen exposure and the body mounting an effective immune response. The delay allows the pathogen time to establish itself, which is why symptoms often appear before the immune system brings the infection under control.
Links to other topics
Defence mechanisms connect to several other biological concepts:
- Cell structure - understanding membrane proteins and receptors
- Cell division - clonal selection involves rapid mitosis
- Protein synthesis - production of antibodies and other immune molecules
- Enzymes - many immune processes involve enzymatic reactions
Key Points to Remember:
- The body uses both non-specific (immediate, general) and specific (slower, targeted) defence mechanisms
- Self vs non-self recognition depends on unique surface proteins that identify different cell types
- Lymphocytes undergo selection during development to prevent attacks on the body's own tissues
- Clonal selection allows rapid multiplication of appropriate lymphocytes when pathogens are detected
- The time lag between infection and immune response occurs because lymphocytes need time to multiply to effective levels