Chapter 21: The Immune System – Innate and Adaptive Body Defenses

Overview of the Immune System

The immune system is a complex network that provides resistance to disease-causing microorganisms. It is a functional system, not a discrete organ system, and consists of multiple lines of defense that work together to protect the body from pathogens.

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

• The immune system is often compared to a medieval castle with multiple lines of defense.

• There are three main lines of defense, encountered by pathogens in sequence:

  • Surface barriers (first line)

  • Innate internal defenses (second line)

  • Adaptive defenses (third line)

Organization of Immune Defenses

Part 1: Innate (Nonspecific) Defenses

• Surface Barriers

• Innate Internal Defenses

Part 2: Adaptive (Specific) Defenses

• Antigens

• B and T Lymphocytes

• Humoral Immunity

• Cellular Immunity

Surface Barriers: The First Line of Defense

Skin and Mucous Membranes

Surface barriers are the body's first line of defense, designed to keep invaders out. These include the skin and mucous membranes, along with their secretions.

• Keratinized epidermis: The outer layer of skin is heavily keratinized, making it highly effective against most microbes. Keratin is a protein resistant to weak acids, bases, and bacterial enzymes.

• Mucosae: Mucous membranes provide similar mechanical barriers within the body, lining the digestive, respiratory, and other tracts.

Protective Chemicals Produced by Surface Barriers

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

• Enzymes: Lysozyme in saliva, respiratory mucus, and lacrimal fluid kills many microbes. Protein-digesting enzymes in the stomach destroy pathogens.

• Mucin: A sticky protein that forms mucus, trapping microbes in the digestive and respiratory tracts.

• Defensins: Broad-spectrum antimicrobial peptides secreted in response to breaches 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 removal.

If surface barriers are breached (e.g., by cuts), the internal innate defenses are triggered to protect deeper tissues.

Summary Table: Lines of Defense

Line of Defense	Components	Main Function
First Line	Skin, mucous membranes, secretions	Prevent entry of pathogens
Second Line	Phagocytes, NK cells, inflammation, antimicrobial proteins, fever	Inhibit spread of invaders; nonspecific internal defense
Third Line	B and T lymphocytes, antibodies	Specific defense against particular pathogens

Key Terms and Concepts

• Pathogen: A microorganism that can cause disease.

• Innate Immunity: Nonspecific defense mechanisms present from birth; includes barriers and internal defenses.

• Adaptive Immunity: Specific defense mechanisms that develop after exposure to antigens; involves lymphocytes and antibodies.

• Antigen: Any substance that can provoke an immune response.

• Lymphocytes: White blood cells (B cells and T cells) central to adaptive immunity.

Example: The skin acts as a physical and chemical barrier, preventing most bacteria and viruses from entering the body. If a cut occurs, the innate internal defenses (such as phagocytes and inflammation) are activated to prevent infection.

Additional info: The immune system's organization into innate and adaptive components allows for both rapid, nonspecific responses and slower, highly specific responses to pathogens. This layered defense is crucial for effective protection against a wide variety of infectious agents.