(20) (C3.2) - Defence Against Infectious Disease - IB Biology (SL/HL)

The Immune System Overview

• The immune system is a complex defense mechanism of the body against pathogens, which are disease-causing organisms.

• It is essential for survival, helping protect the body from infections.

Major Components

• Systems in the Body: Multiple systems (like cardiovascular and respiratory) work together to accomplish tasks.

• Definition of Pathogen: Any disease-causing organism that can infect the body.

Types of Pathogens

• Non-Living Pathogens:

  • Viruses: Infect cells by hijacking their machinery to replicate themselves.

• Living Pathogens:

  • Bacteria: Can be both harmful and beneficial; pathogenic bacteria cause diseases.

  • Fungi: Includes organisms that can cause infections like athlete's foot.

  • Parasites: Include organisms like worms, which can cause significant health issues.

First Line of Defense

• Skin: Acts as a physical barrier against pathogens; includes two main layers:

  • Epidermis: The outermost layer consisting largely of dead cells.

  • Dermis: The inner layer containing blood vessels, nerves, and immune cells.

• Mucous Membranes: Found in openings like the mouth and nose; they produce mucus to trap pathogens and have immune functions.

• Cohesive and Adhesive Properties of Mucus: Mucus traps pathogens, preventing them from entering the body.

Importance of First Line of Defense

• Pathogens must successfully breach these barriers to cause infection.

• Self-Isolation: Preventive measure to avoid pathogen exposure.

Blood Clotting Mechanism

• Injury Response: When the skin is cut, the body initiates a clotting process to seal the wound:

  • Platelets Detect Damage: They sense cuts and release clotting factors.

  • Formation of Prothrombin to Thrombin: Clotting factors activate prothrombin, converting it to thrombin.

  • Fibrinogen Activation: Thrombin converts fibrinogen (soluble) into fibrin (insoluble), forming a fibrous mesh that stops bleeding.

• Platelet Plug: Collection of platelets and fibrin that prevents further blood loss.

Innate Immunity

• Definition: Non-specific defense mechanism that responds to all pathogens identically.

• Key Players: Macrophages (large white blood cells) engulf and destroy pathogens through:

  • Endocytosis: Process of intake; specifically phagocytosis for large particles like bacteria.

• Self vs. Non-Self Recognition: T-cells help recognize and activate B-cells for specific responses; non-self cells marked by antigens.

Adaptive Immunity

• Characteristics: Specific immune response to distinct pathogens, providing long-term protection and memory.

• B-cells and T-cells: Key cells in adaptive immunity.

  • B-cells: Produce antibodies after activation.

  • T-cells: Assist in B-cell activation and recognition of pathogens.

• Memory Cells: Formed after initial infection, providing faster responses upon re-exposure to the same pathogen.

Antibodies

• Nature: Proteins specific to certain antigens; are produced by plasma cells.

• Mechanisms of Action:

  • Neutralization: Blocking pathogens' effects.

  • Opsonization: Marking pathogens for destruction by macrophages.

  • Agglutination: Clumping pathogens together to enhance clearance.

Vaccination and Herd Immunity

• Vaccination: Introduces antigens to develop immunity without causing disease.

• Herd Immunity: Protects entire communities by reducing the spread of infectious diseases when a sufficient percentage of the population is vaccinated.

Impact of HIV on the Immune System

• HIV: Human Immunodeficiency Virus attacks T-cells, specifically CD4 T-lymphocytes, leading to immunodeficiency.

• AIDS: Acquired Immunodeficiency Syndrome is the long-term consequence of HIV infection, making individuals susceptible to opportunistic infections.

• Transmission: Occurs through blood, sexual contact, and from mother to child during pregnancy and breastfeeding.

Summary

• The immune system comprises innate and adaptive responses to protect against various pathogens.

• Key processes include blood clotting and the formation of specific immune responses through B and T cells.

• HIV significantly compromises the immune system, leading to AIDS and increased vulnerability to infections.