Nonspecific Defenses of the Host

Overview

Nonspecific (innate) defenses are the body's first line of protection against invading pathogens. These mechanisms are present from birth and do not require prior exposure to a pathogen. They include mechanical, chemical, and biological factors that work together to prevent infection and disease.

A. Mechanical Factors

Mechanical barriers physically block or remove pathogens from the body, reducing the risk of infection.

• Skin: The skin acts as a dry, tightly packed layer that is inhospitable to most microbes. Its low moisture, high keratin content, and constant shedding make it an effective barrier.

• Mucous Membranes: These line body cavities and trap pathogens. Cilia and mucus help sweep away microbes, especially in the respiratory and gastrointestinal tracts.

• Flushing Actions: Tears, saliva, and urine constantly wash potential pathogens away from the body.

• Other Actions: Coughing, sneezing, and vomiting help expel pathogens from the body.

Example: The mucociliary escalator in the respiratory tract moves trapped particles upward to be expelled or swallowed.

B. Chemical Factors

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

• Skin Secretions: Sebum and sweat lower skin pH, discouraging pathogen colonization.

• Salt Accumulation: Salt from sweat inhibits bacterial growth.

• Lysozyme: An enzyme found in tears, saliva, and mucus digests peptidoglycan in bacterial cell walls, especially effective against Gram-positive bacteria.

• Acidic Environments: The low pH of the stomach and vagina prevents colonization by many pathogens.

• Lactoferrin: A protein in secretions binds iron, making it unavailable to microbes that need it for growth.

Example: Gastric acid in the stomach destroys ingested microbes before they can reach the intestines.

C. Biological Factors

Biological defenses involve cellular and molecular mechanisms that identify and eliminate pathogens.

• Phagocytosis: White blood cells (especially neutrophils and macrophages) engulf and digest foreign particles and microbes.

• Natural Killer (NK) Cells: These cells recognize and destroy infected or abnormal host cells, such as those infected by viruses or transformed by cancer.

• Interferons: Proteins released by virus-infected cells that signal neighboring cells to heighten their antiviral defenses.

• Fever: Elevated body temperature can inhibit microbial growth and enhance immune responses. Fever is regulated by the hypothalamus in response to pyrogens.

• Complement Proteins: A group of plasma proteins that, when activated, enhance phagocytosis, cause cell lysis, and promote inflammation. The classical pathway involves antibodies binding to antigens, triggering complement activation.

Example: Activated complement proteins can cause the release of histamine, attract phagocytes (chemotaxis), act as opsonins to enhance phagocytosis, and form membrane attack complexes that lyse cells.

Complement System Effects

• Release of histamine (inflammation)

• Chemotaxis (attracts phagocytes)

• Opsonization (enhances phagocytosis)

• Cytolysis (causes cell lysis)

Interferons

• Interferons are signaling proteins that help protect uninfected cells from viral infection.

Additional info:

• Phagocytosis Equation: The rate of phagocytosis can be described as:

• Complement Activation: The classical pathway is initiated by antigen-antibody complexes.

• Fever Regulation: Pyrogens stimulate the hypothalamus to increase body temperature.