Human Body Defense Mechanisms

• Pathogens must access a portal of entry, penetrate the surface, and evade the body’s defenses to cause disease in humans.
• Innate and adaptive defenses are the two broad categories protecting the human body.
• Innate barriers are the body's first line of defense.
• The innate immune response is the body's second line of defense.
• The adaptive immune response is the body's third line of defense.

Surface Environment

• The environment on body surfaces inhibits microbes from causing infection through factors like nutrients, temperature, and pH.
• Barriers are nonspecific and act as the first line of defense.
• Structural, mechanical, chemical, and genetic factors make up these barriers.

Structural Barriers

• Epithelium, found internally and externally, is a structural surface barrier.
• Skin and mucous membranes are the two types of epithelial surfaces.
• Epithelium undergoes constant mitosis, replacing sloughed layers with microbes.
• Tight junctions exist between epithelial cells.

Mechanical Barriers

• These barriers involve movement to physically remove microbes.
• Mucociliary movement lines the nose and throat, trapping microbes in mucus and moving them to be coughed or swallowed.
• Fluid movement from urine, saliva, and tears continually flushes microbes from the surface.

Chemical Barriers

• Chemical barriers are agents produced by surface cells.
• Keratin keeps skin dry and waterproof.
• Perspiration secreted by sweat glands contains salt to inhibit pathogen growth.
• Lysozymes produced by saliva and tears have antibacterial properties.
• Sebum secreted by sebaceous (oil) glands lowers skin pH, inhibiting many bacteria.
• Hydrochloric acid (HCL) in the stomach lowers pH.

Genetic Barriers

• Genetic barriers mean a host is genetically immune to diseases of other hosts.
• The microbe lacks appropriate receptors or the ability to attack host cells, known as species specificity.
• Examples of species specificity are HIV versus FIV, Ebola virus, and heartworms.

Normal flora

• Normal flora are capable of surviving due to several adjustments to adhere to the surface, using adhesins.
• Adhesins bind to complementary receptor molecules in a lock-and-key fit.

Terms

• Salt, lysozymes, mucociliary movement, HCL, urine, and lactobacillus are all examples of barriers.
• Mitosis is a structural barrier.
• Pinworms, vomiting, structural, chemical, mechanical, and genetic factors are all related to the line of defense.
• The mouth, nose, skin, intestinal tract, urinary tract, and reproductive tract are portals of entry.

Second Line of Defense

• The second line of defense activates when pathogens penetrate the skin or mucous membranes.
• Inflammation, phagocytosis, interferon, and complement are the four components involved with the blood in the innate immune response.

Inflammation

• Inflammation is a nonspecific response to tissue damage from trauma, wounds, or burns.
• Goals include removing the causative agent, keeping damage local, and facilitating healing.
• Redness, heat, swelling, and pain characterize inflammation.
• Loss of function is a fifth sign, caused by pain and swelling.
• Following injury, vascular changes, edema (local), and fever (systemic) occur sequentially.

Vascular Changes

• Chemicals released trigger dilation.
• Prostaglandins and histamine cause vasodilation, producing redness and localized heat.
• Blood clotting proteins and leukocytes are delivered to the injury site.

Edema

• Increased permeability is caused by the same chemicals.
• Swelling dilutes toxins and applies pressure to nearby nerves.

Fever

• Pyrogens trigger the hypothalamus to increase the body's core temperature.
• Bacterial toxins (exogenous) and phagocytes (endogenous) are types of pyrogens.
• Fever inhibits microbe growth and enhances phagocytosis.

Inflammation

• Neutrophils and monocytes are delivered to the site of infection due to vascular changes and edema.
• Chemotactic factors recruit neutrophils and monocytes.
• Diapedesis is when neutrophils and monocytes squeeze between vessel wall cells to enter the infection site.
• Phagocytosis allows for tissue repair and pathogen removal if present.

Phagocytosis

• Phagocytes are cells capable of phagocytosis.
• Chemotaxis involves recognition of the invader due to signal molecules on the microbe surface.
• Ingestion is when membrane extensions surround the microbe, forming a phagosome.
• Fusion happens when a lysosome fuses with a phagosome to create a phagolysosome.
• Killing occurs when digestive enzymes in the lysosome destroy the microbe.
• Elimination is the discharging of debris out of the cell.

Chemical Defenses

• Interferon are protein molecules released by host cells to nonspecifically inhibit the spread of viral infections.
• Interferon inhibits reproduction of the virus and is released by a cell once infected to protect neighboring cells.

Complement

• The complement system consists of serum proteins designated numerically by their order of discovery.
  • C3a and C5a cause inflammation.
  • C3b causes opsonization.
  • C5B-9 causes MAC/cytolysis.