The 3 Lines of Immune Defense

First Line Defenses

• Definition: Prevent microbes from entering the body.
• Characteristics: Non-specific; always function the same way.
• Types:
  • Physical:
    • Intact skin acts as a barrier.
    • Outward flow of liquids (sweat, urine, mucus) prevents microbial entry.
    • Hairs, mucous coat, and cilia trap microbes in the respiratory tract.
    • Reflexes (cough, sneeze, vomit, blink) expel microbes.
  • Chemical:
    • Lysozyme breaks down peptidoglycan to prevent bacterial infections.
    • Acids (sweat, stomach, vagina) create hostile environments for pathogens.
    • Antimicrobial chemicals are secreted by various glands and tissues, including semen.

Second Line Defenses

• Definition: Cells, tissues, and organs that protect the body from pathogens.
• Components:
  • Phagocytosis: White blood cells (WBCs) engulf and destroy pathogens.
    • Neutrophils are an example of WBCs that can engulf bacteria.
  • Inflammatory response:
    • Triggered by tissue injury.
    • Signs: redness, heat, swelling, pain.
  • Complement system proteins: Enhance the immune response and directly destroy pathogens.
  • Interferons: Proteins that warn neighboring cells of viral infections.
  • Fever: Speeds up the immune response and inhibits pathogen growth.

Third Line Defenses

• Definition: Adaptive or acquired immunity; the body's most specific and strongest response to infection.
• Components:
  • T Lymphocytes:
    • T Helper Lymphocytes: Activate a specific immune response.
    • Cytotoxic T Lymphocytes: Kill infected cells to prevent pathogen spread.
  • B Lymphocytes: Produce antibodies.
  • Memory cells: B and T cells that remain after infection to quickly recognize and respond to the pathogen if it returns.
  • Antibodies: Target and neutralize pathogens or alert WBCs for destruction.

First Line Defenses

• Physical barriers block entry of microbes

  • Intact skin acts as a physical barrier, preventing microbes from entering the body
    • The keratin layer on the skin effectively blocks most microbes
    • Burn patients are at high risk of infection due to the loss of the skin barrier
  • Outward flow of liquids from body openings, such as sweat, urine, saliva, and mucous, acts as a physical barrier against microbes
  • Respiratory tract defenses include hairs, mucous, and cilia:
    • Hairs trap large particles, while mucous traps smaller microbes and debris
    • Cilia beat constantly to move mucous out of the nose and mouth
  • Reflexes, including coughing, sneezing, vomiting, and blinking, expel microbes that attempt to enter the body

• Chemical defenses prevent microbial entry

  • Lysozyme, an enzyme found in body secretions (tears, saliva, sweat, semen, cervical secretions), breaks down peptidoglycan, preventing bacterial infections
  • Acids in body tissues (sweat, stomach acid, vaginal lactic acid) create an environment unfavorable for pathogens
  • Antimicrobial chemicals, such as those found in semen, protect against microbes

Second Line Defenses

• Phagocytosis: White blood cells (WBCs) engulf and destroy pathogens

  • Neutrophils are a type of WBC that targets bacteria in a non-specific manner
  • Phagocytes act generally and cannot target specific pathogens or launch an immune response

• Inflammatory response: The body's way to prevent infection spread, remove damaged tissue, and call for immune reinforcements

  • Occurs when an area of the body is injured, leading to:
    • Redness (due to blood vessel dilation in the area)
    • Heat (due to increased blood flow)
    • Swelling (due to fluid accumulating in tissue)
    • Pain (due to tissue damage, swelling, and chemical mediators that increase nerve sensitivity)

• Complement system proteins: Enhance the immune response and directly destroy pathogens

• Interferons: Proteins that warn neighboring cells of viral infection, protecting them

• Fever: Speeds up the immune response of WBCs and B cell antibody production

  • May inhibit pathogen growth and division

Third Line Defenses

• Adaptive or Acquired Immunity: The body's most specific and powerful defense against infection
  • Occurs when microbes bypass first and second line defenses
  • Depends on T and B lymphocytes to target and destroy specific pathogens
• T Helper Lymphocytes activate specific immune response
• Cytotoxic T Lymphocytes kill infected self-cells to prevent pathogen spread
• B Lymphocytes produce antibodies and can activate T Helper cells
• Memory cells (B and T cells) remain after infection, allowing the body to recognize and respond rapidly to subsequent exposures to the same pathogen
• Antibodies target and bind to pathogens to neutralize them or alert WBCs for destruction

Second and Third Line Defenses

• Second and Third line defenses are internal defenses
  • They work together to fight against pathogens that successfully bypass first line defenses
  • Make up the body's immune system, essential for fighting infection and disease

Neutrophils; The First Responders

• Neutrophils are the most abundant type of white blood cell, accounting for 60% of our total white blood cell count.
• They are the primary responders to bacterial and fungal infections.
• Neutrophils are phagocytic cells, meaning they engulf and destroy foreign invaders like bacteria.
• Neutrophils navigate to the site of infection through a process called chemotaxis, following chemical signals released by damaged cells and pathogens.
• Neutrophils are short-lived cells, unable to renew their cellular structures. They die after engulfing a limited number of bacteria.
• The accumulation of dead neutrophils and other cellular debris forms pus at the site of infection or injury.

Explore the first and second line defenses of the immune system. Learn how physical barriers and various immune responses protect the body from pathogens through both non-specific and specific mechanisms. Test your understanding of the components involved in these critical defense strategies.