The Immune System: Innate and Adaptive Body Defenses

The Immune System: Overview

Introduction to the Immune System

The immune system is a complex network of cells, tissues, and organs that protects the body from disease-causing microorganisms such as bacteria, viruses, and other pathogens. It provides immunity through multiple lines of defense, each with distinct mechanisms and roles.

• Immunity: The ability of the body to resist infection and disease.

• The immune system operates through innate (nonspecific) and adaptive (specific) defenses.

Lines of Defense

The immune system is organized into three main lines of defense:

1. First Line of Defense: Surface Barriers

   • Consists of intact skin and mucous membranes, which act as structural barriers to prevent pathogen entry.

2. Second Line of Defense: Innate Internal Defenses

   • Activated when the first line is penetrated.

   • Includes inflammation and internal defenses such as antimicrobial proteins and phagocytes.

   • Provides a general, nonspecific response to limit the spread of invaders.

3. Third Line of Defense: Adaptive (Specific) Defense System

   • Mounts a specific response to particular pathogens.

   • Response is slower to develop but highly targeted.

Integration of Innate and Adaptive Defenses

Cooperation Between Systems

Innate and adaptive immune systems are integrated and communicate through shared molecules and signaling pathways.

• Both systems release and recognize many of the same defensive molecules.

• Innate responses have specific pathways to target foreign substances.

• Proteins released during innate responses alert adaptive immune cells to the presence of specific foreign molecules.

• Effective immune system function protects against most infectious microbes and cancer cells.

Surface Barriers

Role of Skin and Mucous Membranes

Surface barriers are the body's first line of defense, consisting of the skin and mucous membranes, along with their secretions. These barriers are highly effective at preventing the entry of most microbes.

• Keratinized epidermis: Provides a tough, protective layer against microbial invasion.

• Mucosae: Line internal tracts and offer similar mechanical protection.

Protective Chemicals Produced by Barriers

• Acid: The acidity of skin, vaginal, and stomach secretions inhibits bacterial growth, forming an acid mantle.

• Enzymes: Lysozyme in saliva, respiratory mucus, and tears destroys many microbes; protein-digesting enzymes in the stomach kill pathogens.

• Mucin: Sticky mucus (from the protein mucin dissolved in water) lines the digestive and respiratory tracts, trapping microbes.

• Defensins: Broad-spectrum antimicrobial peptides secreted in response to barrier breach and inflammation; inhibit microbial growth.

• Other chemicals: Lipids in sebum and dermcidin in sweat are toxic to bacteria.

Structural Modifications in the Respiratory Tract

• Mucus-coated hairs in the nose trap inhaled particles.

• Cilia in the upper respiratory tract sweep dust- and bacteria-laden mucus toward the mouth for expulsion.

If surface barriers are breached by nicks or cuts, the internal innate defenses are triggered to protect deeper tissues.

Barrier/Secretion	Protective Mechanism	Example
Skin (keratinized epidermis)	Physical barrier	Prevents entry of pathogens
Mucous membranes	Physical and chemical barrier	Traps microbes in respiratory and digestive tracts
Acid mantle	Chemical barrier	Inhibits bacterial growth on skin and in stomach
Lysozyme	Enzymatic destruction	Found in saliva, tears, and mucus
Defensins	Antimicrobial peptides	Secreted in response to infection
Sebum/dermcidin	Chemical toxicity	Toxic to bacteria

Key Terms and Concepts

• Pathogen: Any microorganism that can cause disease.

• Immunity: The ability to resist infection and disease.

• Innate immunity: Nonspecific defense mechanisms present from birth.

• Adaptive immunity: Specific defense mechanisms that develop in response to exposure to antigens.

• Antigen: Any substance that triggers an immune response.

Summary Table: Lines of Defense

Line of Defense	Components	Type of Response
First Line	Skin, mucous membranes, secretions	Physical and chemical barriers (nonspecific)
Second Line	Phagocytes, inflammation, antimicrobial proteins	Internal defenses (nonspecific)
Third Line	B and T lymphocytes, antibodies	Adaptive (specific) immune response

Example: If a person gets a cut on their skin, the breach of the surface barrier allows pathogens to enter. The innate internal defenses, such as inflammation and phagocytes, are then activated to contain and eliminate the invaders.