Innate and active immunity

Innate Immunity Overview

  • Definition: Innate immunity is a nonspecific defense mechanism that responds to pathogens without targeting specific ones.

    • Nonspecific nature: It reacts to common molecular patterns present in various pathogens, like E. Coli, without distinguishing between pathogens.

Mechanism of Action in Innate Immunity

  • Pathogen-associated molecular patterns (PAMPs):

    • Definition: PAMPs are specific molecules found in pathogenic organisms.

    • Response Mechanism: Receptors on cells bind to PAMPs, activating the receptor and initiating an immune response through signal transduction pathways.

  • Signal Transduction Pathways:

    • Overview: Sequence of cellular events triggered by receptor binding, leading to immune responses.

    • Cell Types: Focus on animal cells, though the mechanisms apply broadly across eukaryotic organisms.

Types of Immune Receptors

  • Toll-like Receptors (TLRs):

    • Definition: A major group of receptors extensively studied in innate immunity, embedded in cell membranes.

    • Function: Bind to PAMPs, initiating signal transduction when pathogens approach cells.

  • Cytoplasmic Receptors:

    • Definition: Receptors that float within the cytoplasm of the cell, activating immune responses when pathogens enter the cell.

  • Soluble Receptors:

    • Definition: Receptors floating in blood plasma, capable of dissolving in water, and also involved in detecting pathogens.

Categories of Immune Responses

  • Following the activation of receptors and the initiation of signal transduction, the innate immune response can categorize into three responses:

    1. Production of Nonspecific Defense Molecules

    2. Triggering Inflammation

    3. Activation of the Complement Pathway

1. Nonspecific Defense Molecules

  • Definition: Molecules produced in response to PAMPs that offer general defense against a range of pathogens.

    • Two Main Groups:

    • Antimicrobial Peptides:

      • Definition: Short chains of amino acids that can disrupt pathogen membranes, leading to their death.

      • Function: Attract immune cells or directly cause pathogen cell lysis by compromising their membranes.

    • Signaling Molecules:

      • Function: Recruit white blood cells to the site of infection to facilitate immune response.

2. Inflammation

  • Definition: A physiological response triggered by infected or injured cells.

  • Mechanism:

    • Chemical Release: Involved substances include histamines that cause vasodilation.

    • Vasodilation:

    • Definition: Widening of blood vessels allows for increased blood flow to the affected area.

    • Outcome: More immune cells can move from blood into tissue spaces.

    • Increased Permeability:

    • Results in redness and swelling in the infected area, making it warm to the touch.

    • Promotes the exit of immune cells and other molecules from the bloodstream to target pathogens.

    • Fever Induction:

    • Sometimes, white blood cells will release signals to increase body temperature, which can inhibit pathogen growth.

    • Note: A fever can be beneficial against pathogens, making it inadvisable to suppress it prematurely unless it poses health risks.

3. Complement Pathway Activation

  • Complement System:

    • A series of proteins that work together to enhance the immune response, aiding in the opsonization and lysis of pathogens.

Phagocytic Cells and Natural Killer Cells

  • Phagocytic Cells:

    • Definition: Cells that engulf and digest pathogens.

    • Examples: Macrophages and neutrophils.

    • Function: They locate, engulf

Overview of Vertebrate Innate Immunity

  • Innate Immunity

    • Vertebrates possess an innate immune system.

    • This system responds generally to various molecular patterns associated with pathogens.

  • Response Mechanisms

    • The innate immune system recognizes specific features, known as molecular patterns from pathogens.

    • These patterns can include structural components that are common to many pathogens but not found in host tissues.

  • Pathogen Recognition

    • The immune response initiates upon detecting these molecular patterns, also referred to as pathogen-associated molecular patterns (PAMPs).

    • Examples of PAMPs include:

    • Lipopolysaccharides (LPS) found in the outer membrane of Gram-negative bacteria

    • Peptidoglycan from bacterial cell walls

    • Flagellin, a protein that makes up bacterial flagella

  • General Characteristics

    • Unlike the adaptive immune system, which is highly specific and develops a memory of pathogens, the innate immune system provides immediate defense against a broad range of pathogens without prior exposure.

  • Importance of Innate Immunity

    • This type of immunity serves as the initial line of defense for vertebrates, potentially preventing infections from establishing in the body.

    • It includes various physical barriers (like skin), cellular responses (like the activity of phagocytes), and signaling molecules (like cytokines).

  • Connection to Adaptive Immunity

    • Although the innate immune system is general and non-specific, it plays a crucial role in shaping the adaptive immune response by providing signals and a prompt early defense against pathogens.