Immunology: Non-Specific and Specific Defenses

Overview of Biological Defenses: Specific and Non-Specific Immunities

The human body employs three distinct levels of defense to protect against pathogens and maintain homeostasis. These are categorized into non-specific (innate) and specific (adaptive) defenses.

  • Non-Specific Defenses (Innate Immunity):
    • 1st Line of Defense: Includes mechanical (physical) and chemical barriers that prevent the entry of microbes.
    • 2nd Line of Defense: Activates when pathogens breach the first line. Components include phagocytosis, white blood cells (WBCs), fever, and inflammation.
  • Specific Defenses (Adaptive Immunity):
    • 3rd Line of Defense: Consists of specialized white blood cells and their products designed to target specific pathogens.
      • Antibodies: Proteins that identify and neutralize foreign objects.
      • Specialized T-Cells: Cells that target and destroy infected or cancerous cells.

First Line of Defense: Mechanical (Physical) Barriers

Mechanical barriers represent the physical structures that act as the primary shield against environmental pathogens.

  • The Skin:
    • Epidermis: Composed of stratified squamous epithelium, providing a robust physical shield. It contains Keratin, a waxy protein that acts as a sealant to prevent moisture loss and microbial entry.
    • Dermis: Located beneath the epidermis; it houses White Blood Cells (WBCs) which transition into the second line of defense if the surface is breached.
  • Mucous Membranes:
    • These are multilayered tissues designed for protection. They produce mucus, a substance that keeps the tissue moist and traps potential pathogens.
    • Locations of Mucous Membranes:
      1. Conjunctiva (lining of the eyes).
      2. Lining of the palpebrae (eyelids).
      3. Nasal cavity.
      4. Oral cavity.
      5. Respiratory tract.
      6. Digestive tract.
      7. Urinary tract.
      8. Reproductive tract.
  • Lacrimal Apparatus (Eye Protection):
    • Lacrimal Gland: Produces tears that cleanse the surface of the eye.
    • Lacrimal Ducts: Function to keep the eyeball moist.
    • Lacrimal Sac: Part of the drainage system.
    • Nasolacrimal Duct: Drains tears into the nasal cavity.
    • Chemical Protection: Tears contain lysozymes, enzymes that provide protection against bacterial cell walls.
  • Additional Mechanical Clearances:
    • Saliva: Contains lysozymes and keeps the oral mucous membrane moist.
    • Cilia: Located in the lining of the respiratory tract. These hair-like projections act as an "up escalator," moving mucus and trapped debris away from the lungs.
    • Epiglottis: A flap of elastic cartilage that temporarily closes off the airway when swallowing to prevent food or liquid from entering the respiratory system.

First Line of Defense: Chemical Barriers and Microbial Antagonism

Chemical environments and biological competition further inhibit the growth of pathogens.

  • Body Secretions and pH Levels:
    • Urine: Typically acidic; if the flow is inhibited, it can lead to bacterial infection.
    • Vaginal Secretions: Maintain an acidic pHpH within the vaginal canal, which is lined by a mucous membrane. This acidity is largely due to yeast fermenting glycogen.
    • Sebum: Produced by sebaceous (oil) glands on the skin. It possesses a pHpH level between 33 and 55, creating an inhospitable environment for many microbes.
    • Perspiration (Sweat): Flushes the surface of the skin to wash away potential invaders.
    • Gastric Juices: The stomach produces Hydrochloric acid (HClHCl), which serves to kill microbes ingested with food.
  • Iron Sequestration:
    • The body uses chemicals to bind iron, making it unavailable to pathogens since microbes require iron to replicate.
    • Key iron-binding proteins include: Hemoglobin (Hb), transferrin, and transferritin.
  • Competitive Exclusion (Microbial Antagonism):
    • The Normal Flora (also known as microbiota or the microbiome) consists of beneficial microbes that reside in and on the body. They protect the host by competing with harmful pathogens for nutrients and space, preventing colonisation.

Second Line of Defense: Phagocytosis

Phagocytosis is a non-specific process where specialized cells ingest and destroy foreign particles.

  • Steps of Phagocytosis:
    1. Chemotaxis: The chemical attraction of phagocytes to the site of infection.
    2. Adherence and Migration: Phagocytes leave the systemic circulation to enter the tissue space.
    3. Ingestion: The cell engulfs the microbe.
    4. Vesicle Formation: The microbe is placed within a membrane-bound vesicle called a phagosome.
    5. Fusion: The phagosome fuses with a lysosome (a vesicle containing digestive enzymes) to form a phagolysosome. This is where the actual "digestion" or destruction of the microbe occurs.
    6. Residual Body Formation: The indigestible materials left over are collected into a residual body.
    7. Release: The residual body is discharged from the cell.

Second Line of Defense: White Blood Cells (WBCs)

Different types of leukocytes play specific roles in non-specific immunity.

  • Neutrophils: These cells leave circulation to act as phagocytes. Critically, once they leave the blood vessels, they do not re-enter circulation. Their count (#) increases significantly during acute infections.
  • Eosinophils: These exhibit weak phagocytic activity but are primarily involved in responding to parasitic infections.
  • Monocytes: These cells leave the circulation and transform into macrophages. Their count (#) increases during chronic infections.
    • Wandering Macrophages: These migrate through various tissues, effectively "looking for trouble."
    • Fixed Macrophages: These remain stationary in specific tissues. Examples include those lining the liver sinusoids (Kupffer cells) and those within the alveolar walls of the lungs.

Second Line of Defense: Fever (Pyrexia)

Fever is a systemic non-specific response to infection characterized by an elevation of body temperature.

  • Thermoregulation: The normal body temperature is approximately 98.6∘F98.6^{\circ}F (37∘C37^{\circ}C). The body's "thermostat" is regulated by the hypothalamus in the brain.
  • Mechanism: When cells are injured due to infection, they release chemicals called pyrogens. These pyrogens target the hypothalamus, signaling it to increase the body's set-point temperature.
  • Biological Effects of Fever:
    1. Helps to kill or inhibit the growth of certain microbes.
    2. Increases metabolism (BMR/PHR), which accelerates tissue repair.
    3. Provides more nutrients and O2O_2 to cells to assist in the fight against infection.

Second Line of Defense: Inflammation and Tissue Repair

Inflammation is the body’s localized response to any type of tissue damage.

  • The 5 Cardinal Signs of Inflammation:
    1. Redness (Rubor)
    2. Heat (Calor)
    3. Pain (Dolor)
    4. Edema (Swelling)
    5. Loss of Function
  • Steps in the Inflammatory Process:
    1. Vasodilation: Increased blood flow to the injured area.
    2. Chemical Release: Release of Histamine, which increases the permeability of blood vessels. This increased permeability leads specifically to redness and heat.
    3. Margination: WBCs line up along the inner walls (endothelium) of the blood vessels.
    4. Diapedesis: WBCs squeeze through the capillary walls to exit the bloodstream and enter the damaged tissue spaces.
    5. Kinins: These inflammatory chemicals further increase vessel permeability and contribute to the sensation of pain.
  • Tissue Repair Mechanisms:
    • Stroma: The supporting tissue of an organ, often consisting of connective tissue like collagen.
    • Parenchyma: The functional component or functioning tissue of the organ itself.