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.