Study Notes on the Immune System

HUBS1404 Biomedical Science Part 2: Introduction to the Immune System

Lecture Details

  • Lecture Number: 28

  • Instructor: Dr. Alexandra Spencer

  • Institution: School of Biomedical Sciences & Pharmacy, Faculty of Health, Medicine, and Wellbeing

Learning Objectives

  • Describe the basic components of our immune system.

  • Discuss the lymphatic system.

  • Define different types of white blood cells and their functions.

  • Explain the function of lymph nodes, the spleen, and MALT (Mucosa-Associated Lymphoid Tissue).

Introduction to the Immune System

  • We are perpetually exposed to various pathogens, including:

    • Bacteria

    • Viruses

    • Fungi

    • Parasites

  • Every pathogen employs a distinct method of attack; however, generally, we maintain good health due to our immune system.

  • Humans tend not to perish upon initial exposure to infectious organisms due to established immune responses.

The Importance of Host Immunity

  • The World Health Organization (WHO) estimates that 23% of global deaths are attributable to infectious diseases.

Child Mortality and Pathogen Changes Over Time

  • Data Comparison: Child deaths under 5 years old in 1990 vs. 2017

    • Neonatal Disorders:

    • 1990: 2,986,000

    • 2017: 1,784,000

    • Neonatal Asphyxia & Trauma:

    • 1990: 814,000,

    • 2017: 533,000 (10% of child deaths)

    • Preterm Birth:

    • 1990: —

    • 2017: 649,000 (12% of child deaths)

    • Neonatal Sepsis & Other Infections:

    • 2017: 203,000 (4% of child deaths)

    • Injuries & Accidents:

    • 1990: 635,000,

    • 2017: 270,000

    • Non-communicable Diseases:

    • 1990: 318,000,

    • 2017: 170,000

    • Total Child Deaths from Infectious Diseases:

    • 1990: 6,531,000,

    • 2017: 2,428,000

Evolution of the Immune System

  • Microorganisms preceded animal life, influencing animal evolution.

  • Adaptive immune systems evolved approximately 500 million years ago with the emergence of jawed fish.

Organs of the Immune System: The Lymphatic System

  • Composed of:

    • Lymphatic vessels

    • Lymph

    • Lymphoid cells, tissues, and organs

    • Important components:

      • Skin

      • Thymus

      • Spleen

      • Bone marrow

      • Lymph nodes (Axillary, Inguinal)

      • Peyer's patches (Intestine)

      • Appendix

Functions of the Lymphatic System

  • Captures lost fluid from blood (interstitial fluid, called lymph when in lymphatic system).

  • Transports dietary lipids and foreign substances along with immune cells to lymphoid organs.

Structure of the Lymphatic System

  • Collection of lymph:

    • Flow through ever-larger vessels:

    • Lymphatic capillaries → Collecting vessels → Lymphatic trunks → Ducts

    • Empties into the venous system (Right lymphatic duct and thoracic duct).

Overview of the Immune System

  • Two main divisions:

    • Innate (natural, native) or Non-Specific Immune System

    • Adaptive (Acquired) or Specific Immune System

    • The immune system distinguishes between “self” and “non-self” to target potential threats.

Innate Immune System

  • Present at birth and offers immediate protection against pathogens.

  • General action against pathogens doesn't vary with the number of exposures.

  • Divided into first and second lines of defense.

  • First Line of Defense: Surface barriers such as skin and mucous membranes.

  • Second Line of Defense: Internal defenses including phagocytes, NK cells, antimicrobial proteins, and fever.

Adaptive Immune System

  • Develops specifically in response to foreign objects (antigens).

  • Involves T cells and B cells which recognize specific antigens with unique receptors.

    • Cells have memory, allowing them to respond rapidly to subsequent exposures.

Components of the Immune System

  • White Blood Cells:

    • Neutrophils

    • Eosinophils

    • Basophils/Mast Cells

    • Macrophages

    • Dendritic Cells

    • Natural Killer (NK) Cells

    • Lymphocytes (T cells and B cells)

Neutrophils

  • Also known as Polymorphonuclear Neutrophils (PMNs)

  • Most abundant phagocytes (up to 70% of WBCs).

  • First to arrive at infection sites, chemotactically attracted by chemical gradients.

  • Defense against bacterial infections through generation of reactive oxygen species.

Other Granulocytes

  • Eosinophils:

    • Role in defense against parasitic worms.

  • Basophils:

    • Release histamine and heparin, key roles in inflammation.

Macrophages

  • Large agranular cells derived from monocytes entering tissues.

  • Actively engulf foreign particles and use various ‘weapons’ (e.g., lysozymes, ROS).

  • Engage in both innate and adaptive immune responses by activating lymphocytes and presenting antigens.

Phagocytosis Process

  • Macrophages adhere to pathogens, engulfs them forming a phagosome.

  • The phagosome then fuses with a lysosome to form a phagolysosome, where enzymes digest the particles.

Dendritic Cells

  • Characterized by long 'arms'.

  • Good at capturing antigens and migrating to lymphatic system.

  • Present antigens to T cells in lymph nodes, serving as a major link between innate and adaptive immunity.

Natural Killer (NK) Cells

  • Small population of large granular lymphocytes.

  • Cytotoxic against some cancer and virally infected cells without specific antigen receptors.

  • Recognize defective self antigens and release perforins and granzymes to induce apoptosis.

Lymphocytes

  • B cells and T cells develop in primary lymphoid organs and migrate between secondary lymphatic tissues.

  • Have specific antigen receptors allowing them to recognize and respond to unique antigens.

B Cells

  • Produce antibodies and have unique receptors for specific antigens.

  • Differentiate into plasma cells, producing large amounts of antibodies specific to an antigen.

T Cells

  • Express unique antigen-specific receptors (TCR).

  • Consist of two classes:

    • CD4+ Helper T Cells

    • CD8+ Cytotoxic T Cells.

Recap of Immune Cells

  • Summary of various immune cell types, their lineages, and functions in both innate and adaptive responses.

Lymphoid Tissue and Organs

  • Major components of the immune system containing vast numbers of immune cells.

  • Composed mainly of reticular connective tissue.

  • Includes primary lymphoid organs (bone marrow, thymus) and secondary lymphoid organs (lymph nodes, spleen, MALT).

Key Functions of Lymphoid Organs

Thymus

  • Location for T-cell maturation.

  • Highly active post-birth, remaining active but declining with age.

Lymph Nodes

  • Functions:

    • Filtration of lymph and immune activation.

    • Houses both B and T lymphocytes for immune responses.

Spleen

  • Largest lymphoid organ with two regions (red pulp and white pulp).

  • Roles: Cleansing blood, removing defective blood cells, and recycling products like iron.

Mucosa-Associated Lymphoid Tissue (MALT)

  • Found in mucous membranes in areas like tonsils and gastrointestinal tract.

Immune System Effectiveness

  • Innate immunity reacts immediately (0-4 hours), whereas adaptive immunity takes longer but is highly specific and retains memory for faster responses upon re-exposure.

Summary of the Immune System

  • The lymphatic system is crucial to the immune response through a complex network of vessels, organs, and cells.

  • Immune cells vary in type and function, forming both innate and adaptive responses that work together to eliminate threats to the body.