Chapter 15: Immunity & Body Defenses

• Chapter 15 is about Immunity

Overview of Body Defenses

• Numerous pathogens can cause disease in humans.
• Pathogens must access a portal of entry, penetrate the surface, and evade the body's defenses
• There are two broad categories of defense which are innate and adaptive
• The first line of defense consists of innate barriers
• The second line of defense consists of an innate immune response
• The third line of defense consists of an adaptive immune response
• Innate defenses: Memory doesn't exist, present at birth, and nonspecific.
• Adaptive defenses: Involve memory, specific, and happen simultaneously with innate defenses

The First Line of Defense

• Consists of barriers that are nonspecific
• The environment found on the surface can inhibit microbes from causing infection because of nutrients, temperature, and pH
• Barriers are structural, mechanical, chemical, and genetic barriers

Structural Barriers

• Epithelium is found on the surface both internally and externally.
• Two types of epithelial surfaces exist which are skin and mucous membranes.
• In both instances, the epithelium constantly undergoes mitosis and replaces sloughed layers.
• Microbes attached to the epithelium are lost
• Epithelial cells also have tight junctions between them

Mechanical Barriers

• Typically involve movement to physically remove microbes.
• Mucociliary movement is found lining the nose and throat, where microbes trapped in mucus are carried away by cilia to be coughed or swallowed.
• Fluid movement, like the flow of urine, saliva, or tears, helps continually flush microbes from the surface.

Chemical Barriers

• Consist of agents produced by the cells on the surface
• Keratin keeps the skin dry and waterproof.
• Perspiration is secreted by sweat glands and contains salt, which inhibits the growth of pathogens.
• Lysozymes defend against bacteria and are present in saliva and tears.
• Sebum is secreted by sebaceous (oil) glands, lowering skin pH to an inhibitory level for many bacteria.
• Similarly, pH is lowered in the stomach through the use of HCL.

Genetic Barriers

• Host is genetically immune to diseases of other hosts
• Microbes will not have the appropriate receptors or the ability to attack host cells
• Referred to as species specificity, for example: HIV versus FIV, Ebola virus, and heartworms.

Normal flora survival

• Normal flora make several adjustments to attach (adhere) to the surface of the body
• The attachment is with adhesins that bind to their complementary receptor molecules using a lock and key fit

Second Line of Defense

• Occurs when pathogens penetrate the skin or mucous membranes.
• Is part of the nonspecific innate immune response
• There are four components that involve the blood which are: inflammation, phagocytosis, interferon, and complement

Inflammation

• Inflammation ("itis") is a nonspecific response to tissue damage from various causes such as trauma, wounds, or burns
• Goals include removing the agent causing inflammation, keeping damage local, and allowing for healing to occur
• Prevent inflammation from spreading
• Characterized by redness, heat, swelling, and pain
• The fifth sign of inflammation is loss of function due to pain or swelling
• There are three sequential events that occur following an injury, including vascular changes, edema (local), and fever (systemic)

Vascular Changes

• Many chemicals released trigger dilation, including bradykinins, prostaglandins, and histamine
• Vasodilation can produce redness and localized heat
• Vascular changes delivers blood clotting proteins to site of injury as well as leukocytes

Edema

• Edema(swelling) occurs as the same chemicals released in vascular changes lead to increased permeability.
• Increased pressure is also attributed to the increased pressure of blood
• Swelling dilutes toxins, but can also apply pressure to nearby nerves

Inflammation Summary

• Inflammation comprises the combination of vascular changes and edema
• Neutrophils and monocytes are delivered to the site of infection
• Neutrophils and Monocytes are recruited by chemotactic factors and squeeze between cells of vessel wall and enter site of infection (Diapedesis)
• Phagocytosis occurs allowing for tissue repair and removal of pathogen (if present)

Phagocytosis

• Phagocytosis is where cells are called phagocytes are capable of engulfing a particle
• The process can be divided into five stages:
• Chemotaxis: Recognition of invader due to signal molecules on the microbe surface
• Ingestion: Membrane extensions surround microbe forming a phagosome
• Fusion: Lysosome fuses with phagosome creating a phagolysosome
• Killing: Digestive enzymes in lysosome destroy the microbe
• Elimination: Debris is discharged out of the cell

Fever

• Fever is a systemic factor
• Only occurs when microbe is present
• Results when pyrogens trigger the hypothalamus to increase the body's core temperature
• Two types of pyrogens exist
• Bacterial toxins (exogenous)
• Released by phagocytes (endogenous)
• Fever makes environment less hospitable for the pathogen, inhibiting growth of microbe, enhance phagocytosis

Chemical Defenses: Interferon

• The second line of defense includes chemical defenses
• Interferon consists of protein molecules released by host cells to nonspecifically inhibit the spread of viral infections
• It usually works by inhibiting reproduction of the virus, and is released by a cell once infected to protect neighboring cells

Chemical Defenses: Complement

• Complement includes a set of serum proteins designated numerically according to their order of discovery
• Complement has three primary effects
• C3a C5a contributes to inflammation
• C3b functions in opsonization
• C5b-9 participates in MAC/Cytolysis

Microbe Surface Proteins

• Some microbes produce surface proteins which interferes with direct contact with phagocytes, either due to similarity to host or by blocking receptors.
• These surface proteins can be unique however and allow for attack by the Third Line of Defense.