Immune System: Lines of Defense

Introduction to the Immune System

The immune system protects the body from pathogens through a series of defense mechanisms. These defenses are organized into three main lines, each with distinct roles and components.

• 1st Line of Defense: The initial barrier that pathogens encounter, primarily at the body's surfaces.

• 2nd Line of Defense: Cellular and chemical responses that act when pathogens bypass the first line.

• 3rd Line of Defense: Specific, adaptive responses targeting particular pathogens.

Innate Immune System

First and Second Lines of Defense

The innate immune system provides immediate, non-specific protection against pathogens. It encompasses both the first and second lines of defense.

• First Line:

  • Skin and mucous membranes act as physical and chemical barriers.

• Second Line:

  • Phagocytes: Cells such as neutrophils and macrophages that engulf and digest pathogens (process called phagocytosis).

  • Natural Killer (NK) Cells: Destroy virus-infected cells and tumor cells without antigen specificity.

  • Inflammation: Recruits immune cells and increases blood flow to affected areas.

  • Antimicrobial Proteins: Attack pathogens and inhibit their growth.

  • Fever: Elevates body temperature to inhibit pathogen growth.

Key Innate Immune Cells and Processes

• Phagocytes:

  • Neutrophils: Arrive first at infection sites; less efficient at phagocytosis than macrophages.

  • Macrophages: Highly efficient at phagocytosis; derived from monocytes.

• Natural Killer Cells:

  • Use a two-step process:

    1. Release perforin to perforate the target cell membrane.

    2. Release enzymes that induce apoptosis (programmed cell death).

• Inflammatory Chemicals:

  • Histamines: Released by mast cells and basophils; cause vasodilation.

  • Kinins: Attract white blood cells and induce pain.

  • Prostaglandins: Produced from fatty acids; enhance histamines and kinins.

  • Cytokines: Signal proteins that regulate immunity and inflammation.

• Cardinal Signs of Infection:

  • Redness

  • Swelling

  • Pain

  • Heat

  • Loss of function (sometimes considered a fifth sign)

Adaptive Immune System

Third Line of Defense

The adaptive immune system provides a specific response to pathogens and develops immunological memory. It is responsible for the third line of defense.

• B Cells (Humoral Immunity):

  • Produce antibodies that circulate in blood and lymph.

  • Differentiate into plasma cells (secrete antibodies) and memory B cells (provide long-term immunity).

  • Originate in bone marrow.

• T Cells (Cellular Immunity):

  • Originate in bone marrow, mature in the thymus.

  • Attack infected cells directly.

  • Two main types:

    • Helper T Cells (TH): Assist other immune cells.

    • Cytotoxic T Cells (TC): Kill infected cells; antigen-specific.

Antigen Recognition and Immune Memory

• During first exposure, the body takes time to recognize antigens and produce antibodies through B cell proliferation and differentiation.

• Upon re-exposure, memory cells enable a faster and more effective secondary immune response.

Classification of Immunity

Types of Immunity

Immunity can be classified based on how it is acquired and whether it is active or passive.

Additional Key Concepts

Cellular Communication and Antigen Presentation

• Chemotaxis: Chemical signals that attract immune cells to sites of infection.

• MHC Proteins: Major Histocompatibility Complex proteins are found on the surface of antigen-presenting cells and are essential for immune recognition.

• Antigen Specificity: Cytotoxic T cells are antigen-specific, while natural killer cells are not.

Comparison: Innate vs. Adaptive Immunity

• Innate Immunity: Generalized, immediate response; no memory; includes physical barriers, phagocytes, inflammation, and antimicrobial proteins.

• Adaptive Immunity: Pathogen-specific, slower to activate; develops memory; involves B and T lymphocytes.

Example: When a person is exposed to a virus for the first time, the innate immune system responds immediately, but the adaptive immune system takes time to recognize the virus and produce specific antibodies. Upon subsequent exposures, the adaptive immune system responds more rapidly due to memory cells.

Additional info: Loss of function is sometimes considered a fifth cardinal sign of infection. Cytokines play a crucial role in both innate and adaptive immunity by mediating cell signaling.