Innate Immunity: Nonspecific Defenses of the Host (Chapter 16 Study Guide)

Innate Immunity

Nonspecific Defenses of the Host

Innate immunity refers to the body's first line of defense against pathogens, providing rapid, nonspecific protection. These defenses are present from birth and do not require prior exposure to a pathogen. They include physical, chemical, and biological mechanisms that prevent infection and eliminate invaders.

Physical Factors (Mechanical Barriers)

• Skin: Acts as a physical shield. The outer layer is composed of dry, dead cells that are tightly packed, nutrient-depleted, and inhospitable to microbial growth. The skin's normal flora and high osmotic pressure further discourage colonization.

• Mucous Membranes: Line the respiratory, gastrointestinal, and genital tracts. Mucus traps pathogens and facilitates their removal by washing them away.

• Tears: Constantly wash potential pathogens from the eyes.

• Cilia (Ciliary Escalator): Ciliated cells in the respiratory tract move trapped particles upward toward the throat for removal.

• Saliva: Washes pathogens from the oral cavity.

• Urine: Flushes microbes from the urinary tract.

• Feces: Expels pathogens from the digestive tract.

• Vomiting and Diarrhea: Rapidly remove pathogens from the gastrointestinal tract.

Chemical Factors

• Sebum: Secreted by sebaceous glands on the skin; its low pH discourages pathogen colonization.

• Salt Accumulation: Perspiration evaporates, leaving salt on the skin, increasing osmotic pressure and inhibiting microbial growth.

• Lysozyme: An enzyme found in tears, perspiration, and saliva; digests the peptidoglycan in Gram-positive bacterial cell walls.

• Normal Flora: Microorganisms on the skin and in the vagina reduce pH and prevent colonization of pathogenic microbes in the intestinal and genital tracts.

• Low pH of Stomach: Gastric acid creates a highly acidic environment that is profoundly antimicrobial.

• Transferrin and Lactoferrin: Proteins in blood and human milk that bind free iron, an essential growth factor for microbes, thus limiting microbial proliferation.

Biological Factors

• Phagocytosis: Various white blood cells (leukocytes) ingest and destroy foreign bodies. Key phagocytes include neutrophils, macrophages, and dendritic cells.

• Natural Killer (NK) Cells: Recognize and kill infected host cells or cancer cells by detecting abnormal membrane proteins.

• Inflammation: Damaged host cells release chemical signals that trigger beneficial responses:

  • Vasodilation: Increases blood flow, allowing more immune cells to reach the injured area.

  • Increased Permeability: Blood vessels become more permeable, enabling immune cells and proteins to diffuse into injured tissue.

• Fever: Moderate fever (up to about 104°F) is beneficial. It stimulates immune cell differentiation and growth, inhibits some pathogens (heat shock effect), and encourages rest during infection.

Complement Proteins

The complement system consists of plasma proteins that enhance immune responses. The classical pathway is activated when two antibodies bind to an antigen, causing a conformational change that allows complement proteins to bind and become activated. Activated complement proteins have several effects:

• Release of Histamine: Promotes inflammation.

• Chemotaxis: Attracts phagocytes to the site of infection.

• Opsonization: Complement proteins act as opsonins, enhancing phagocyte attachment and ingestion of pathogens.

• Cytolysis: Causes lysis of target cells.

Interferons

Interferons are signaling proteins released by host cells in response to viral infection. They help inhibit viral replication and activate immune cells.

Summary Table: Nonspecific Defenses of the Host

Defense Type	Examples	Mechanism
Physical	Skin, mucous membranes, cilia, tears, saliva, urine, feces, vomiting, diarrhea	Barrier, removal of pathogens
Chemical	Sebum, salt, lysozyme, normal flora, stomach acid, transferrin, lactoferrin	Inhibits growth, destroys microbes, limits nutrients
Biological	Phagocytosis, NK cells, inflammation, fever, complement, interferons	Destruction of pathogens, immune activation

Key Terms: innate immunity, phagocytosis, natural killer cells, inflammation, fever, complement proteins, interferons, sebum, lysozyme, transferrin, lactoferrin

Example: When a person inhales a pathogen, mucus and cilia in the respiratory tract trap and remove the invader, while lysozyme in saliva and tears further break down bacterial cell walls. If the pathogen breaches these barriers, phagocytes and complement proteins work together to destroy it.

Additional info: The complement system can also be activated by alternative and lectin pathways, not just the classical pathway. Interferons are classified as alpha, beta, and gamma, each with specific roles in antiviral defense and immune regulation.