Innate Immunity | Immune System

Introduction to the Immune System

  • Focus on the innate division of the immune system

Two Divisions of the Immune System

Adaptive Immune System

  • Specific response division

  • Targets specific pathogens (bacteria, viruses, fungi)

  • Responds uniquely to each pathogen

  • Analogy: hiring specialized guards to respond to different types of attackers

Innate Immune System

  • Non-specific response to pathogens

  • Does not differentiate between types of attackers

  • Analogy: physical barriers (walls, doors, windows) that universally obstruct intruders

  • Key Characteristics:

    • Non-specific

    • No memory of previous encounters

    • Acts as the first line of defense

Components of the Innate Immune System

External Division

  • Chiefly involves skin and mucous membranes

  • Classification: First line of defense, non-specific, and without memory

Skin

  • Epidermis: Most superficial layer made of stratified squamous epithelia (many layers of squished dead cells)

  • Functions:

    • Protects underlying tissues

    • Sebaceous oil glands secrete oils with:

      • Unsaturated fatty acids (poor energy source for microbes)

      • pH between 3-5 (inhospitable for microbes)

  • Sweat: Contains water and salt (not ideal for microbial growth)

  • Hair: Traps particles, combined with mucus for further protection

Mucous Membranes and Secretions

  • Tears and Saliva: Contain enzymes like defensins and lysozymes that destroy pathogens

  • Cilia in Airways: Move trapped pathogens in mucus to be expelled or swallowed

  • Stomach Acid: Gastric secretions with pH of 1-3 denature proteins of pathogens

Internal Division of the Innate Immune System

Components of Internal Defenses

  1. Cellular Responses

    • Phagocytes (cells that eat pathogens): include neutrophils and macrophages

    • Neutrophils:

      • Most abundant white blood cells

      • Typically target bacterial pathogens

    • Macrophages:

      • Can be wandering (monocytes that migrate to tissue) or fixed types (histiocytes, Kupffer cells, microglia)

      • Act as antigen-presenting cells (APCs), aiding the adaptive immune response

  2. Natural Killer (NK) Cells

    • Destroy infected or cancerous cells without specificity

    • Release perforins (to create holes in pathogens) and granzymes (inducing apoptosis in target cells)

  3. Chemical Responses

    • Complement Proteins: Activate a cascade response enhancing innate defense functions

    • Cytokines: Interleukins and interferons that strengthen both innate and adaptive immunity; assist in recruiting other immune cells

  4. Physiological Responses

    • Inflammation: Response to vascularized tissue damage, involves chemical mediators (prostaglandins, histamine, bradykinins)

      • Vasodilation and increased permeability to allow immune cell access to affected area

      • Cardinal signs of inflammation: redness, heat, pain, swelling

    • Fever: Induced by pyrogens affecting hypothalamic temperature regulation

      • Enhances immune function and limits pathogen reproduction

Summary

  • The innate immune system serves as a crucial first defense mechanism against various pathogens through external barriers (skin, mucous membranes) and internal cellular and chemical responses.

  • It operates non-specifically and does not retain memory of past infections, but its effectiveness is vital to maintaining health.