Innate Immunity | Immune System

Immune System Overview

  • Two main divisions: adaptive and innate immune systems.

    • Adaptive Immune System:

      • Specific response to pathogens (bacteria, viruses, fungi).

      • Responds in tailored ways to target, attack, and eliminate pathogens.

    • Innate Immune System:

      • Non-specific response; reacts the same way regardless of the pathogen.

      • Acts as the first line of defense.

Innate Division Characteristics

  • Non-specificity: Does not differentiate between pathogens.

  • No Memory: Once it targets and attacks a pathogen, it does not remember it for future encounters.

  • Acts as the first line of defense, akin to barriers that prevent intrusion into a house.

Components of the Innate Immune System

  • External Division:

    • Includes skin and mucous membranes.

    • Skin:

      • Composed of the epidermis, the outermost layer made of stratified squamous epithelium (many layers of flattened cells).

      • Dead cells are protective, minimizing damage when sloughed off.

      • Sebaceous Oil Glands:

        • Produce oil containing unsaturated fatty acids, a poor energy source for microbes.

        • Maintain pH of 3-5, creating an inhospitable environment for bacteria and viruses.

      • Sweat:

        • Primarily water and salt, both flush away microbes and create a salty environment that deters pathogens.

      • Hair and Mucus:

        • Traps airborne pathogens and particles.

      • Tears and Saliva:

        • Contain enzymes (defensins and lysozymes) that break down pathogens.

Internal Division of Innate Immunity

  • When pathogens enter the bloodstream, they trigger internal defenses which include:

    • Cellular Defenses:

      • Phagocytes (cells that "eat" pathogens):

        • Neutrophils: Most abundant white blood cells that engulf and destroy mainly bacteria.

        • Macrophages: Can be wandering (e.g., monocytes) or fixed in tissues (e.g., Kupffer cells in the liver).

        • Present antigens to activate adaptive immune response (APCs).

    • Natural Killer (NK) Cells:

      • Target and destroy infected cells regardless of pathogen type.

      • Release perforins (create pores in pathogen membranes) and granzymes (induce apoptosis).

    • Chemical Responses:

      • Complement Proteins: 30+ proteins that activate in a cascade to support immune functions and enhance the role of phagocytic cells and inflammation.

      • Cytokines: Chemical signals (such as interleukins and interferons) that can recruit immune cells to sites of infection and modulate immune responses.

Physiological Responses in Innate Immunity

  • Inflammation:

    • Triggered by damage to vascularized tissues (has a blood supply).

    • Releases chemicals (prostaglandins, histamine, bradykinin) leading to vasodilation and increased vascular permeability.

    • Cardinal signs: redness, heat, pain, swelling.

    • Necessary for attracting immune cells to the site and promoting tissue repair.

  • Fever:

    • Induced by pyrogens (pathogens or cytokines) acting on the hypothalamus.

    • Increases body temperature to enhance immune cell function and inhibit pathogen reproduction.

    • Can result in mechanisms like shivering to create heat in response to a perceived drop in body temperature.

Summary of Key Points

  • Innate immune system comprises both external (skin, mucous membranes) and internal (cellular defenses) responses.

  • Non-specific and without memory, acts quickly upon pathogen invasion.

  • Plays a crucial role in complementing the adaptive immune system, supporting efficient pathogen clearance and promoting healing.