Chapter 21: The Immune System – Study Notes

Chapter 21: The Immune System

Overview of the Immune System

The immune system is a complex network of cells, tissues, and molecules that defends the body against pathogens and abnormal cells. It consists of innate (nonspecific) and adaptive (specific) defenses, which work together to protect the body from infection and disease.

• Innate Defenses: Surface barriers, cells, and chemicals that provide immediate, nonspecific protection.

• Adaptive Defenses: Antigen-specific responses involving lymphocytes and the production of antibodies.

• Imbalance and Development: Disorders and developmental aspects of the immune system.

Innate (Nonspecific) Defenses

First Line of Defense: Surface Barriers

Surface barriers are the body's initial defense against invading pathogens, primarily through the skin and mucous membranes.

• Skin: Provides a physical barrier that is impermeable to most microorganisms.

• Mucous Membranes: Line body cavities and secrete mucus to trap pathogens.

• Protective Chemicals:

  • Acid: Skin and stomach acidity inhibit microbial growth.

  • Enzymes: Lysozyme in saliva, tears, and respiratory mucus destroys bacteria.

  • Mucin: Traps microorganisms in digestive and respiratory tracts.

  • Defensins: Antimicrobial peptides that inhibit microbial growth.

  • Other Chemicals: Sebum and dermcidin are toxic to bacteria.

• Respiratory System Modifications: Mucus-coated hairs in the nose and cilia in the upper respiratory tract trap and remove pathogens.

Second Line of Defense: Internal Defenses

If pathogens bypass surface barriers, internal defenses are activated, including various cells and chemicals.

• Phagocytes: Cells that engulf and digest pathogens (e.g., neutrophils and macrophages).

• Natural Killer (NK) Cells: Lymphocytes that target and destroy virus-infected and cancerous cells.

• Inflammatory Response: Localized tissue response to injury or infection, characterized by redness, heat, swelling, pain, and loss of function.

• Antimicrobial Proteins: Interferons and complement proteins that inhibit microbial reproduction and directly attack pathogens.

• Fever: Systemic response that raises body temperature to inhibit microbial growth and enhance immune function.

Phagocytosis Mechanism

1. Phagocyte adheres to the pathogen.

2. Engulfs the pathogen, forming a phagosome.

3. Phagosome fuses with a lysosome, forming a phagolysosome.

4. Enzymes digest the pathogen.

Some pathogens resist digestion by inhibiting phagolysosome formation or surviving within phagocytes.

Natural Killer (NK) Cells

• Attack cells lacking "self" cell-surface receptors.

• Induce apoptosis in cancer and virus-infected cells.

• Secrete chemicals that enhance the inflammatory response.

Inflammatory Response

• Triggered by tissue injury or infection.

• Prevents spread of damaging agents, disposes of cell debris, and sets the stage for repair.

• Cardinal Signs: Redness, heat, swelling, pain, and loss of function.

• Inflammatory Mediators: Kinins, prostaglandins, and complement proteins.

• Edema: Increased capillary permeability leads to fluid leakage and swelling.

Phagocyte Mobilization

1. Leukocytosis: Release of neutrophils from bone marrow.

2. Margination: Neutrophils cling to capillary walls.

3. Diapedesis: Neutrophils move through capillary walls.

4. Chemotaxis: Inflammatory chemicals attract neutrophils to the site of injury.

Antimicrobial Proteins

• Interferons (IFNs): Proteins released by virus-infected cells to warn neighboring cells and inhibit viral replication.

• Complement Proteins: Circulate in inactive form and become activated to enhance inflammation and destroy pathogens.

Fever

• Systemic response to infection, raising body temperature.

• Moderate fever helps sequester iron and zinc, increases metabolic rate, and accelerates tissue repair.

• High fever can be dangerous due to enzyme denaturation.

Adaptive (Specific) Defenses

Overview of Adaptive Immunity

The adaptive immune system provides specific defense against pathogens and abnormal cells. It requires initial exposure to antigens and has memory for stronger responses upon re-exposure.

• Specificity: Recognizes and targets specific antigens.

• Systemic: Not restricted to the initial infection site.

• Memory: Mounts a stronger attack to previously encountered antigens.

Arms of Adaptive Immunity

• Humoral Immunity: Antibody-mediated; antibodies circulate in body fluids and bind to extracellular targets (e.g., bacteria, toxins).

• Cellular Immunity: Cell-mediated; lymphocytes (mainly T cells) act against infected or abnormal cells.

Antigens

Antigens are substances that trigger an immune response. Most are large, complex molecules not normally found in the body.

• Complete Antigens: Have immunogenicity (stimulate lymphocyte proliferation) and reactivity (react with activated lymphocytes and antibodies).

• Incomplete Antigens (Haptens): Small molecules that are only immunogenic when attached to body proteins.

Summary Table: Innate vs. Adaptive Immunity

Feature	Innate Immunity	Adaptive Immunity
Specificity	Nonspecific	Specific to antigens
Response Time	Immediate	Slower (days to weeks)
Memory	No memory	Has memory
Main Components	Barriers, phagocytes, NK cells, inflammation, fever, antimicrobial proteins	B and T lymphocytes, antibodies

Key Terms and Definitions

• Pathogen: A microorganism that causes disease.

• Phagocyte: A cell that engulfs and digests foreign particles.

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

• Antibody: A protein produced by B cells that binds to a specific antigen.

• Cytokine: A signaling molecule that modulates immune responses.

• Complement System: A group of proteins that enhance immune responses and destroy pathogens.

• Interferon: A protein that interferes with viral replication.

• Inflammation: A localized response to injury or infection.

• Fever: An elevated body temperature in response to infection.

Formulas and Equations

• Osmotic Pressure (related to edema):

$\pi = iMRT$

• Rate of Phagocytosis (conceptual):

$\text{Rate} = k[\text{Phagocytes}][\text{Pathogens}]$

Examples and Applications

• Example of Innate Immunity: Skin preventing entry of bacteria after a minor cut.

• Example of Adaptive Immunity: Production of specific antibodies after vaccination.

• Application: Interferons are used therapeutically to treat viral infections such as hepatitis C and some cancers.

Additional info: These notes summarize the main points from the provided slides, expanding on key concepts and providing context for college-level Anatomy & Physiology students.