Innate Immunity: The First Line of Defense

Innate Immunity

Overview of the Immune System

The immune system protects the body from infectious agents through two main defense systems: innate immunity and adaptive immunity. Innate immunity provides immediate, nonspecific defense against pathogens, while adaptive immunity offers a specific response that develops later.

• Innate Immunity (Nonspecific Defenses): Includes first and second line defenses, such as physical barriers, chemical barriers, and cellular responses.

• Adaptive Immunity (Specific Defenses): Involves third-line defenses, including antibodies and cytotoxic cells, which target specific pathogens.

First Line of Defense: Physical and Chemical Barriers

Skin: Physical Structure

The skin acts as a primary physical barrier to microbial invasion. It is composed of two main layers:

• Epidermis: Multiple layers of tightly packed cells, with the outermost layers consisting of dead cells that are regularly shed, removing attached microbes. Dendritic cells within the epidermis play a role in immune surveillance.

• Dermis: Contains collagen fibers that provide strength and flexibility, helping the skin resist abrasions that could introduce microbes.

Example: A cut through the skin can provide a route for pathogens to enter, bypassing the protective barrier.

Skin: Chemical Defenses

The skin produces several chemicals that inhibit microbial growth:

• Perspiration: Contains salt, which inhibits pathogen growth, and lysozyme, which destroys bacterial cell walls.

• Sebum: Secreted by sebaceous glands, keeps skin pliable and lowers pH, inhibiting many bacteria.

• Antimicrobial peptides: Small proteins that act against a variety of microbes.

Mucous Membranes: Physical Structure

Mucous membranes line all body cavities open to the environment and serve as another important barrier:

• Epithelium: Thin, living outer layer of tightly packed cells that prevents pathogen entry. Contains goblet cells (produce mucus) and ciliated columnar cells (move mucus and trapped invaders out).

• Deeper connective layer: Produces chemicals that defend against pathogens.

Comparison of Skin and Mucous Membranes

The following table compares the main features of skin and mucous membranes as barriers to infection:

Feature	Skin	Mucous Membrane
Number of Cell Layers	Many	One to a few
Cells Tightly Packed?	Yes	Yes
Cells Dead or Alive?	Outer layers: dead; inner layers: alive	Alive
Mucus Present?	No	Yes
Relative Water Content	Yes	Yes
Lysozyme Present?	Yes	With some
Sebum Present?	Yes	No
Cilia Present?	No	Trachea, uterine tubes
Constant Shedding and Replacement of Cells?	Yes	Yes

Lacrimal Apparatus

The lacrimal apparatus protects the eyes by producing and draining tears. Blinking spreads tears and washes the surface of the eye, while lysozyme in tears destroys bacteria.

Role of the Microbiome in Innate Immunity

The normal microbiota (microbiome) provides protection through microbial antagonism, making it difficult for pathogens to establish themselves:

• Compete with pathogens for nutrients and attachment sites

• Create an environment unfavorable to other microbes (e.g., by altering pH)

• Stimulate the body's second line of defense

• Produce antimicrobial compounds

• Promote overall health by providing vitamins and modulating immunity

Other Secretions and Chemicals

Various organs secrete chemicals with antimicrobial properties, contributing to the first line of defense:

• Antimicrobial peptides: Present in skin, mucous membranes, and neutrophils; act against a variety of microbes in several ways.

• Other secretions: Many organs produce chemicals that inhibit or destroy microbes.

Secretions and Activities of Body Systems

The following tables summarize the secretions and activities of different body systems that contribute to the first line of defense:

System	Secretion/Activity	Function
Digestive	Saliva	Washes microbes from oral surfaces; contains lysozyme
Digestive	Stomach acid	Digests/inhibits microorganisms
Digestive	Gastroferritin	Sequesters iron, making it unavailable for microbes
Digestive	Bile	Inhibitory to most microorganisms
Digestive	Intestinal secretions	Digest/inhibit microorganisms
Digestive	Peristalsis	Moves GI contents, eliminating pathogens
Digestive	Defecation/Vomiting	Eliminates microorganisms

System	Secretion/Activity	Function
Urinary	Urine	Contains lysozyme; acidity inhibits microbes; washes microbes from tract
Reproductive	Vaginal secretions	Acidity inhibits microbes; contains iron-binding proteins
Reproductive	Menstrual flow	Cleanses uterus and vagina
Reproductive	Prostate secretion	Contains iron-binding proteins
Cardiovascular	Blood flow	Removes microorganisms from wounds
Cardiovascular	Coagulation	Prevents pathogen entry

Summary Questions and Key Points

• Mucus-secreting membranes are found in all body cavities open to the environment, including the urinary, digestive, and respiratory systems.

• The skin is inhospitable to most microbes due to salt, defensins, enzymes, and low pH, but some normal microbiota are present.

• Normal microbiota protect the body by consuming nutrients, altering pH, and keeping the immune system stimulated, but do not directly attack and kill pathogens.