3.3. Describe the body’s defence mechanisms against infection

This guide will help you answer 3.3. Describe the body’s defence mechanisms against infection.

The human body has evolved complex systems to protect against infections. These defences are essential in maintaining health and well-being. Understanding these mechanisms can help you appreciate how your body fights off harmful invaders.

First Line of Defence: Physical and Chemical Barriers

Skin

The skin acts as a physical barrier. It is your body’s first defence against infection. The skin is tough and prevents pathogens—harmful microorganisms—from entering.

• Epidermis: The outer layer is made of dead cells. It constantly sheds, removing potential invaders.
• Sebum: An oily substance produced by glands that keeps the skin supple. It also contains fatty acids that lower pH, deterring bacteria.

Mucous Membranes

Mucous membranes line body cavities. They are found in the respiratory, digestive, and urogenital tracts. These membranes trap pathogens.

• Mucus: A sticky substance that captures dust, bacteria, and viruses.
• Cilia: Hair-like structures in the respiratory tract. They move mucus towards the throat for expulsion.

Chemical Barriers

The body secretes various substances that inhibit bacterial growth.

• Stomach Acid: Hydrochloric acid in the stomach kills most pathogens.
• Lysozyme: An enzyme found in tears, saliva, and sweat. It breaks down bacterial cell walls.

Second Line of Defence: Innate Immune System

Phagocytosis

Phagocytes are a type of white blood cell that engulf and digest pathogens. This process is called phagocytosis.

• Macrophages: Large phagocytes that reside in tissues.
• Neutrophils: Fast-acting phagocytes that respond quickly to infections.

Inflammatory Response

When tissues are injured by bacteria, trauma, or toxins, inflammation occurs. This response aims to eliminate the initial cause of cell injury.

• Heat and Redness: Increased blood flow to the area.
• Swelling: Accumulation of fluid.
• Pain: Chemicals like histamine intensify the sensation of pain, alerting you to the injury.

Fever

A rise in body temperature can inhibit bacterial growth. Fever also boosts the effectiveness of the immune system.

• Pyrogens: Substances that cause fever by affecting the hypothalamus in the brain.

Third Line of Defence: Adaptive Immune System

Unlike the innate system, the adaptive immune response is specific. It targets particular pathogens and provides long-term immunity.

Lymphocytes

These are specialised white blood cells that play an essential role in the adaptive immune system.

T Cells

• Helper T Cells: Activate other immune cells.
• Cytotoxic T Cells: Destroy infected or cancerous cells.

B Cells

• Plasma Cells: Produce antibodies, proteins that recognise and neutralise foreign invaders.
• Memory B Cells: Retain information about pathogens for quicker response in future infections.

Antibodies

Antibodies attach to antigens on pathogens, marking them for destruction. They are highly specific to each pathogen.

• Structure: Y-shaped proteins with variable regions that bind to specific antigens.
• Functions: Neutralise toxins, agglutinate (clump together) microbes, enhance phagocytosis.

The Complement System

The complement system consists of proteins in the blood that aid the immune system.

• Opsonisation: Coating of pathogens to enhance phagocytosis.
• Lysis: Direct destruction of bacteria by puncturing cell walls.
• Inflammation: Increasing the inflammatory response to attract more immune cells.

The Role of the Lymphatic System

The lymphatic system is a network that transports lymph, a fluid containing infection-fighting white blood cells.

• Lymph Nodes: Filter lymph, trapping bacteria, and viruses.
• Spleen: Removes old red blood cells and provides a site for immune responses.

The Role of Vaccination

Vaccination is a way to train the adaptive immune system. It exposes the body to a harmless form of a pathogen, prompting an immune response without causing illness.

• Herd Immunity: When a large portion of a community becomes immune, it reduces the spread of disease.
• Booster Shots: Enhance or renew immunity for certain diseases.

Final Thoughts

The body’s defence mechanisms are multifaceted and work in harmony to protect against infections. From physical barriers like skin to complex immune responses, each component plays an important role. Understanding these defences can empower you to maintain your health and appreciate the body’s resilience.