Body's Defenses: Immunity and Barriers

Overview of Body's Defenses

• Numerous pathogens can cause disease in humans, requiring access, surface penetration, and defense evasion.
• Innate and adaptive immunity are the two broad categories of defense.
• The first line of defense involves innate barriers.
• The second line of defense involves innate immune responses.
• The third line of defense involves adaptive immune responses.
• Innate immunity does not involve memory, is present at birth, and is non-specific.
• Adaptive immunity involves memory, appears later in life and is specific to pathogens.
• Both defenses are fully protective, meaning both are needed at the same time to be fully effective.

Body's First Line of Defense

• The environment on the body's surface inhibits microbes from causing infection.
• Barriers that microbes must pass include nutrients, temperature, and pH.
• These nonspecific barriers serve as the first line of defense.
• These barriers can be structural, mechanical, chemical, and genetic.

Structural Barriers

• Epithelium is present both internally and externally on the body's surface.
• Skin and mucous membranes are the two types of epithelial surfaces.
• The constant mitosis of epithelium replaces sloughed layers, which both instances are constantly going through.
• Attached microbes are lost, due to sloughing and there are tight junctions between epithelial cells.

Mechanical Barriers

• Mechanical barriers typically use movement to physically remove microbes.
• Mucociliary movement involves mucus-trapped microbes in the nose and throat being coughed/swallowed.
• Fluid movement involves the flow of urine/saliva/tears flushing microbes away.

Chemical Barriers

• Agents produced by cells on the surface act as chemical barriers.
• Keratin keeps the skin dry and waterproof.
• Perspiration, secreted by sweat glands, contains salt inhibiting pathogen growth.
• Lysozymes are antibacterial and are made by saliva and tears.
• Sebum, secreted by sebaceous (oil) glands, lowers skin pH, inhibiting bacteria.
• HCL lowers the pH in the stomach.

Genetic Barriers

• Hosts are genetically immune to diseases of other hosts.
• Microbes may lack appropriate receptors or the ability to attack host cells.
• Species specificity describes this phenomenon.
• Examples include HIV versus FIV, Ebola virus, and heartworms.

Normal Flora Survival

• Normal flora survive barriers through adjustments.
• Attachment occurs via adhesins.
• Adhesins bind to complementary receptor molecules.
• Adhesins and receptors have a lock and key fit.

Practice Examples

• Salt is an integument that performs a chemical barrier.
• Lysozymes are respiratory and perform a chemical barrier.
• Mucociliary movement is respiratory and a mechanical barrier.
• HCL is digestive and provides a chemical barrier
• Urine (Urinary) a chemical barrier
• Lactobacillus is an epidermis
• Mitosis (inner, structural)
• Pinworms intestinal, genetic
• Vomiting digestive, mechanical

Body's Second Line of Defense

• The second line of defense activates when pathogens penetrate the skin or mucous membranes, that are considered initial barriers.
• The second line is still part of the nonspecific innate immune response.
• It consist of four components: Inflammation, Phagocytosis, Interferon, and Complement.
• All four components have involvement with the blood

Inflammation

• Inflammation is a nonspecific response to tissue damage.
• Sources for tissue damage can be from trauma, wounds, or burns.
• The four goals of inflammation are to remove the agent causing damage, keep the damage local, and allow for healing.
• Redness, heat, swelling, and pain are the four signs of inflammation.
• The fifth sign is loss of function resulting form pain/swelling

Inflammation Events

• Following an injury, inflammation occurs.
• Vascular changes, edema, and fever are the three sequential events of inflammation.

Vascular Changes

• Many chemicals released during injury trigger dilation.
• Bradykinins, Prostaglandins, and Histamine all cause dilation.
• Vasodilation produces redness and localized heat.
• Vasodilation delivers blood clotting proteins and leukocytes to the injury site.

Edema

• Increased vascular permeability during inflammation can lead to edema.
• Chemicals lead to increased permeability.
• Increased pressure from more blood occurs.
• Swelling dilutes toxins, although swelling applies pressure to nearby nerves.

Inflammation Actions

• Neutrophils and monocytes are delivered to the site of infection.
• Chemotactic factors recruit these cells.
• Cells squeeze between vessel walls and enter the infection site, called diapedesis.
• Phagocytosis occurs, allowing for tissue repair and pathogen removal.

Phagocytosis

• Phagocytes are cells capable of phagocytosis.
• The process can be divided into five stages (in order):
  • Chemotaxis: recognition of an invader due to signal molecules on the microbe surface
  • Ingestion: membrane extensions surround microbes, forming a phagosome
  • Fusion: a lysosome fuses with the phagosome, creating a phagolysosome
  • Killing: digestive enzymes in the lysosome destroy the microbe
  • Elimination: debris is discharged out of the cell

Fever

• Fever is a Systemic factor that measures 107 degrees.
• Only if a microbe is present does the body experience a fever.
• When pyrogens trigger the hypothalamus, the body's temperature increases.
• Pyrogens have various types that include Bacterial toxins (exogenous) and released from phagocytes (endogenous).
• Fever inhibits microbe growth and enhances phagocytosis, making it less hospitable for pathogens.

Chemical Defenses: Interferon

• Chemical defenses use interferon
• Protein molecules released by host cells nonspecifically inhibit the spread of viral infections.
• Interferon typically works by inhibiting the reproduction of the virus.
• When a cell is infected it will release cells to protect its neighbors.

Chemical Defenses: Complement

• Complements consists of a set of serum proteins numerically designated according to their order of discovery
• Complements will have 3 primary effects:
  • C3a C5a: inflammation
  • C3b: opsonization
  • C5b-9: MAC/Cytolysis

Microbe Interference

• Certain microbes producing Surface Proteins, have the capability to interfere with the direct contact of phagocytes.
• It can occur due to similarity to the host or from blocking receptors.
• Third Line of Defense however, allows for attack of unique surface proteins.