Innate and Adaptive Immune Defenses: An Overview

Immune System: General Overview

Introduction to the Immune System

The immune system is a functional system that provides resistance to disease. It is not a single organ system, but rather a collection of cells, tissues, and molecules working together to defend the body against pathogens (disease-causing microorganisms).

• The immune system is divided into two intrinsic systems: innate (nonspecific) defenses and adaptive (specific) defenses.

• These systems are intertwined and often work together to protect the body.

Innate (Nonspecific) Defense System

Definition and Function

The innate defense system provides the first and second lines of defense against pathogens. It responds quickly and is present from birth, offering general protection rather than targeting specific invaders.

• First line of defense: External body membranes such as the skin and mucous membranes.

• Second line of defense: Internal mechanisms including antimicrobial proteins, phagocytes, natural killer cells, inflammation, and fever.

• Innate defenses recognize common features of pathogens using pattern recognition receptors.

First Line of Defense: Surface Barriers

Surface barriers are the body's initial physical and chemical defense against pathogens.

• Skin: Acts as a tough, physical barrier resistant to weak acids, bases, bacterial enzymes, and toxins.

• Mucous membranes: Line body cavities open to the exterior and produce protective chemicals.

• Secretions: Include chemicals that inhibit or destroy microorganisms.

Protective Chemicals of Surface Barriers

• Acid mantle: The acidity of skin and some mucous secretions inhibits microbial growth.

• Enzymes: Lysozyme in saliva, respiratory mucus, and tears destroys bacteria; stomach enzymes kill many microorganisms.

• Mucin: Sticky mucus that traps microorganisms in the digestive and respiratory tracts.

• Defensins: Antimicrobial peptides that inhibit microbial growth.

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

Respiratory System Modifications

• Mucus-coated hairs: In the nose, these trap inhaled particles.

• Cilia: In the upper respiratory tract, cilia sweep dust- and bacteria-laden mucus toward the throat for removal.

Summary Table: Innate vs. Adaptive Defenses

Innate Defenses	Adaptive Defenses
Surface barriers (skin, mucous membranes) Internal defenses (phagocytes, natural killer cells, inflammation, antimicrobial proteins, fever)	Humoral immunity (B cells) Cellular immunity (T cells)

Key Terms and Definitions

• Pathogen: A microorganism that can cause disease.

• Phagocyte: A type of white blood cell that engulfs and digests foreign invaders.

• Antimicrobial protein: Proteins that attack microorganisms or hinder their ability to reproduce.

• Inflammation: A localized response to injury or infection, characterized by redness, heat, swelling, and pain.

• Fever: An elevated body temperature that enhances immune function and inhibits pathogen growth.

Additional info:

• The innate and adaptive immune systems communicate through the release of signaling molecules such as cytokines.

• While the innate system responds rapidly and broadly, the adaptive system is slower but highly specific and has memory for previously encountered pathogens.