Host Defenses I: Mechanisms of the Immune System

Mechanisms of the Host: An Overview

Introduction to Host Defenses

The human body is protected by a multilevel network of innate and adaptive immune mechanisms. These defenses are commonly classified into three lines of defense, each contributing to comprehensive protection against microbial invasion.

• Innate immunity refers to non-specific defenses present from birth.

• Adaptive immunity refers to specific defenses developed in response to particular pathogens.

• The three lines of defense work in concert to ensure survival against microbial threats.

The Three Lines of Defense

First Line of Defense

The first line of defense consists of physical and chemical barriers that prevent the entry of pathogens into the body.

• Physical barriers: Skin, mucous membranes of the respiratory, digestive, urinary, and reproductive tracts.

• Chemical barriers: Lysozyme in tears and saliva, acidic pH of the stomach, fatty acids on the skin.

• Microbiota barrier: Normal microbiota compete with pathogens for resources and space.

Second Line of Defense

The second line of defense is composed of innate immune mechanisms that act rapidly once pathogens breach the first line.

• Phagocytosis: The process by which certain white blood cells (e.g., neutrophils, macrophages) engulf and destroy microbes.

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

• Fever: An increase in body temperature that can inhibit the growth of some pathogens.

• Antimicrobial products: Substances such as interferons, complement proteins, and antimicrobial peptides that target and neutralize pathogens.

Third Line of Defense

The third line of defense is the adaptive immune response, which is highly specific and provides long-term immunity.

• Lymphocytes: Specialized white blood cells, including B cells and T cells, that recognize and respond to specific antigens.

• Immunological memory: The ability of the adaptive immune system to remember previous encounters with pathogens and respond more effectively upon re-exposure.

Summary Table: Three Lines of Defense

Line of Defense	Definition/Functions	Characteristics
First line	A surface protection composed of anatomical and physiological barriers that keep microbes from penetrating sterile body components	Physical barriers Microbiota barrier Chemical barriers
Second line	A cellular and chemical system that comes immediately into play if infectious agents make it past the surface defenses	Phagocytosis Inflammation Fever Antimicrobial products
Third line	Includes specific host defenses that must be developed uniquely for each microbe through the action of specialized white blood cells	B and T cells

Levels of Defense: Implications for Human Systems

Integration and Overlap of Defenses

Human immune defenses are not completely separated; they overlap and cooperate to eliminate microbial invaders. This redundancy increases the effectiveness of the immune response.

• Immunology is the study of the immune system and its functions, especially the second and third lines of defense.

• A healthy immune system is responsible for:

  1. Surveillance of the body

  2. Recognition of foreign material

  3. Destruction of entities deemed foreign

Antigens & Antibodies

Antigens (Immunogens)

An antigen is any cell, particle, or chemical that induces a specific immune response. Antigens stimulate resistance to infection or toxins.

• Body cells are recognized as "self"; foreign materials (e.g., bacteria) are "non-self".

• Autoimmune disorders occur when the immune system mistakenly attacks self tissues.

Antibodies

An antibody is a large protein molecule produced in response to an antigen. It binds specifically to that antigen, neutralizing or marking it for destruction.

The Lymphatic System

Fluid Compartments

The body is partitioned into several fluid-filled spaces: intracellular, extracellular, cerebrospinal, and circulatory compartments. Coordination among these compartments is essential for immune responsiveness.

Lymphatic System Structure and Function

• The lymphatic system is a network of vessels, cells, and specialized organs.

• Main functions:

  • Surveillance

  • Recognition

  • Protection against foreign materials via lymphocytes and phagocytes

• Primary lymphatic organs: Sites where immune cells are born (e.g., bone marrow, thymus).

• Secondary lymphatic organs: Sites where immune cells reside, become activated, or carry out their functions (e.g., lymph nodes, spleen).

The Circulatory and Lymphatic Systems

Blood and Lymph

• The circulatory system consists of the heart, arteries, veins, and capillaries that circulate blood.

• The lymphatic system includes lymphatic vessels and organs (lymph nodes) that circulate lymph.

• Whole blood consists of blood cells suspended in plasma.

• Serum is plasma without clotting factors; it is often used in immune testing and therapy.

First Line of Defense: Physical Barriers

Physical Barriers

Physical barriers impede the entry of microbes and other foreign substances.

• Skin: The outer layer (stratum corneum) is composed of tightly packed epithelial cells, forming an effective barrier.

• Mucous membranes: Line the digestive, urinary, respiratory, and reproductive tracts; mucus traps and removes microbes.

• Other mechanisms: Urine flow, vaginal secretions, and periodic cleansing help prevent infection.

Second Line of Defense: Innate Immune Mechanisms

Main Mechanisms

The second line of defense includes four main mechanisms:

1. Phagocytosis

2. Inflammation

3. Fever

4. Antimicrobial products

Phagocytosis

• General purpose phagocytes (e.g., neutrophils, macrophages) engulf and destroy pathogens.

• Neutrophils accumulate rapidly at sites of injury or infection.

Inflammatory Response

• Triggered by infection, trauma, or immune reactions.

• Main functions:

  1. Destroy microbes and block further invasion

  2. Mobilize and attract immune components

  3. Repair tissue damage

• Helps maintain stability after injury.

Fever

• Abnormally elevated body temperature, often a systemic component of inflammation.

• Common in infection, allergies, cancers, and other conditions.

• Caused by pyrogens that reset the hypothalamic thermostat.

• Can inhibit temperature-sensitive microbes (e.g., poliovirus, herpes zoster).

Antimicrobial Products

• Interferons (IFN): Target viruses and some cancer cells.

• Complement: Targets membranes of pathogens and infected host cells.

• Antimicrobial peptides: Directly kill a variety of microbes.

• Restriction factors: Inhibit multiplication of viruses.

Summary of Host Defense Mechanisms

The host defense network is organized into three lines of defense:

• First line: Physical barriers, microbiota barrier, and chemical barriers (e.g., lysozyme, fatty acids, pH).

• Second line: Innate mechanisms (phagocytosis, inflammation, fever, antimicrobial products).

• Third line: Adaptive responses (B and T cells, immunological memory).

Example: When a pathogen enters through a cut in the skin, the first line (skin barrier) is breached, the second line (inflammation and phagocytosis) responds rapidly, and the third line (adaptive immunity) develops a specific response if the pathogen persists.

Additional info: The notes have been expanded to include definitions, examples, and context for each mechanism, as well as a summary table for clarity.