Nonspecific Defenses of the Host

Introduction

Nonspecific (innate) defenses are the first line of protection against pathogens and are present from birth. These defenses do not target specific pathogens but provide broad protection through physical, chemical, and biological mechanisms. Understanding these defenses is essential for appreciating how the body resists infection before the adaptive immune system is activated.

A. Mechanical Factors

Mechanical factors are physical barriers and processes that prevent pathogens from entering or remaining in the body.

• Skin as a shield: The skin's dry, dead outer layer is inhospitable to microbes due to nutrient depletion, tightly packed cells, normal flora, and high osmotic pressure.

• Mucous membranes: These line the respiratory, gastrointestinal, and genital tracts, trapping and washing away pathogens.

• Tears: Constantly wash potential pathogens away from the eyes.

• Ciliary escalator: Cilia in the respiratory tract move mucus and trapped microbes upward, away from the lungs.

• Saliva: Constantly washes potential pathogens from the mouth.

• Urine: Flushes microbes from the urinary tract.

• Feces: Expel pathogens from the digestive tract.

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

B. Chemical Factors

Chemical factors create hostile environments for pathogens or directly destroy them.

• Sebum: Secreted on skin, contains fatty acids and has a low pH, discouraging pathogen colonization.

• Salt accumulation: From sweat evaporation, increases osmotic pressure and discourages microbial growth.

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

• Normal flora: Reduce pH of skin and vagina, preventing colonization of intestinal and genital tracts by pathogens.

• Low pH of stomach: Highly acidic environment (pH ~2) is strongly antimicrobial.

• Transferrin and lactoferrin: Proteins in blood and milk bind iron, making it unavailable for microbial growth (iron is an essential growth factor for many microbes).

C. Biological Factors

Biological factors involve cellular and molecular responses that destroy or inhibit pathogens.

• Phagocytosis: Performed by various white blood cells (leukocytes), which ingest and destroy microbes.

• Inflammation: Damaged host cells release chemical factors that trigger a beneficial response:

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

  • Increased permeability: Allows immune cells and proteins to diffuse into tissues.

• Fever: Moderate fever (up to about 104°F) is beneficial, stimulating immune cell differentiation and growth, and inhibiting some pathogens. Fever also induces rest, which aids recovery. Excessive fever can be harmful.

D. Complement Proteins

The complement system consists of plasma proteins that enhance immune responses. The classical pathway is activated when antibodies bind to antigens, causing a conformational change that allows complement proteins to bind and become activated.

• Functions of activated complement proteins:

  • Cause release of histamine, leading to inflammation.

  • Produce chemotaxins, attracting phagocytes to the site of infection.

  • Act as opsonins, coating pathogens to enhance phagocytosis.

  • Induce cytolysis, causing cells to lyse.

E. Interferons

Interferons are proteins produced by host cells in response to viral infection. They help protect neighboring cells by inducing antiviral states and activating immune cells.

Summary Table: Nonspecific Defenses of the Host

Key Terms and Definitions

• Pathogen: A microorganism that can cause disease.

• Phagocytosis: The process by which cells ingest and destroy foreign particles.

• Opsonization: The coating of pathogens by proteins to enhance phagocytosis.

• Cytolysis: The destruction of cells by lysis.

• Interferon: A protein released by host cells in response to pathogens, especially viruses.

• Complement system: A group of proteins that enhance immune responses and destroy pathogens.

Relevant Equations

• Osmotic Pressure Equation:

• pH Calculation:

Additional info: Some explanations and terms have been expanded for clarity and completeness, including definitions and the summary table.