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  • Microbiota Role in Defense

    • Microbiota provide a natural defense mechanism against pathogens as they occupy niches, consume available resources, and secrete antimicrobial substances.
    • This concept is known as microbial antagonism, wherein beneficial microbes prevent the colonization of harmful ones.

  • First Line of Defense (Innate Immunity)

    • Physical Barriers:
    • Skin: Acts as a tough physical barrier against invasion. Keratinocytes make the skin impenetrable for many microbes.
    • Mucous Membranes: Found in moisture-rich areas like the nose, mouth, and genital tract. These membranes secrete mucus that traps pathogens.
    • Chemical Defense:
    • Tears and Saliva: Contain lysozyme, which breaks down bacterial cell walls.
    • Gastric Acid: Extremely acidic environment kills microorganisms in the stomach.
    • Urinary Tract: Continuous flushing helps prevent infections like UTIs.

  • Second Line of Defense (Innate Immunity)

    • Involves the action of immune cells and substances produced in response to any tissue injury. This includes:
    • Phagocytosis: Process where pathogens are engulfed by immune cells like macrophages and neutrophils.
    • Inflammation: Characterized by redness (rubor), heat (calor), swelling (tumor), pain (dolor). This helps contain infections and initiate healing.
    • Fever: Elevated body temperature inhibits pathogen growth and enhances immune response.

  • Third Line of Defense (Adaptive Immunity)

    • B and T Lymphocytes: Specialize in targeting specific pathogens.
    • B Lymphocytes (B cells): Responsible for antibody-mediated immunity. They produce antibodies to neutralize pathogens.
    • T Lymphocytes (T cells): Activate and coordinate the immune response. Subtypes include:
      • Helper T cells: Assist other immune cells.
      • Cytotoxic T cells: Kill infected host cells.

  • Inflammation Process

    • Triggered by tissue injury or infection.
    • Cardinal Signs: Rubor, calor, tumor, dolor.
    • Involves the movement of immune cells (chemotaxis) to the site of infection and the formation of pus through the accumulation of dead cells, pathogens, and immune cells.

  • Complement System

    • A complex series of proteins that enhances the ability of antibodies and phagocytic cells to clear pathogens.
    • Activated via three pathways:
    • Classical Pathway: Triggered by antibodies binding to pathogens.
    • Lectin Pathway: Initiated when certain proteins bind to specific sugars.
    • Alternative Pathway: Activated by the presence of pathogens directly.
    • Functions include opsonization (marking pathogens for destruction), lysis (bursting pathogens), and inflammation enhancement.

  • Cells of the Immune System

    • White Blood Cells (Leukocytes): Key players in the immune response, categorized into:
    • Granulocytes: Contain granules with enzymes; include neutrophils, eosinophils, basophils.
    • Agranulocytes: Include lymphocytes (B and T cells) and monocytes (which differentiate into macrophages and dendritic cells).
    • Phagocytes: Such as neutrophils and macrophages, which engulf and digest pathogens.

  • Antigen Recognition and Response

    • Antigen-presenting cells (APCs) display antigens to T cells to initiate a specific immune response.
    • The recognition of pathogens and subsequent immune response involves MHC molecules (Major Histocompatibility Complex) that present antigens to T cells.
    • Activation of B cells leads to antibody production, while T cells can directly kill infected cells or activate other immune cells.

  • Key Features of Immune System Functioning

    • Immunological Memory: Once exposed, the immune system remembers pathogens allowing faster responses to subsequent exposures.
    • Understanding these defenses clarifies the body's complex interaction in maintaining health and responding to threats.