Innate Immunity: The First and Second Lines of Defense

Crash Course in Immunology: Part I

Learning Objectives

• Differentiate innate and adaptive immunity.

• Describe the role of chemical & physical factors in innate immunity as a first line of defense.

• Describe the role of defensive cells in innate immunity as a second line of defense.

• Know the signs & symptoms of inflammation, its purpose, and its basic stages, as a second line of defense in innate immunity.

• Describe the role of fever, complement, and interferons in innate immunity as a second line of defense.

Overview of Immunity

Types of Immunity

The immune system protects the body from pathogens through two major types of immunity: innate and adaptive.

• Innate Immunity:

  • Present from birth

  • Always available and provides immediate protection

  • Nonspecific, general, and fast response

  • Includes physical and chemical barriers, defensive cells, and certain physiological responses

• Adaptive Immunity:

  • Develops more slowly

  • Provides a later, highly specific defense

  • Stimulated by exposure to specific pathogens

  • More effective and involves memory cells

Innate Immunity

First Line of Defense

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

Physical Factors

• Skin: Forms a physical barrier to the entrance of microbes. The outer layer (epidermis) is composed of tightly packed cells and is often impermeable to pathogens.

• Mucous Membranes: Line the respiratory, gastrointestinal, and genitourinary tracts; inhibit entrance of microbes.

• Mucus: Sticky secretion that traps microbes and particles.

• Cilia: Hair-like structures that propel microbes upward and out of the lungs, aiding in their removal.

Chemical Factors

• Lysozyme: Enzyme found in tears, saliva, and nasal secretions; digests peptidoglycan in bacterial cell walls, leading to cell lysis.

• Gastric Juice: Highly acidic (low pH) secretion in the stomach that destroys microbes and toxins.

Second Line of Defense

If pathogens bypass the first line of defense, the second line of defense is activated, involving defensive cells and physiological responses.

Defensive Cells

• Phagocytes:

  • Migrate to the site of infection

  • Ingest and digest microbes

  • Types include neutrophils, dendritic cells, and macrophages

• Natural Killer (NK) Cells:

  • Destroy tumor and virus-infected cells

  • Release perforin (creates pores in target cell membranes) and granzymes (induce apoptosis, or programmed cell death)

Inflammation

Inflammation is a localized response to cell damage, characterized by four cardinal signs:

• Heat

• Redness

• Swelling

• Pain

Functions of inflammation:

• Confines and destroys microbes

• Initiates tissue repair

The Process of Inflammation (Example: Skin)

1. Tissue Damage: Physical injury allows bacteria to enter the skin.

2. Vasodilation, Increased Permeability, and Blood Clotting: Chemicals such as histamine and cytokines are released, causing blood vessels to dilate and become more permeable; blood clot forms to contain the infection.

3. Phagocyte Migration and Phagocytosis: Phagocytes migrate to the site, adhere to the endothelium, and ingest invading bacteria.

4. Tissue Repair: Regeneration of epidermis (parenchyma) and dermis (stroma) occurs, restoring tissue integrity.

Fever

• Inhibits growth of microbes by raising body temperature above the optimal range for many pathogens.

• Speeds up repair reactions by increasing metabolic rate.

• Disadvantages: High fever can cause tachycardia, dehydration, seizures, and even death at extreme temperatures (112-114°F).

Antimicrobial Substances

• Complement System:

  • Group of serum proteins that enhance ("complement") the immune response

  • Functions include direct lysis of pathogens, opsonization (marking for phagocytosis), chemotaxis (attracting immune cells), and inflammation

• Interferons:

  • Antiviral proteins secreted by infected cells

  • Signal uninfected cells to protect themselves by producing antiviral proteins

Comparison Table: Innate vs. Adaptive Immunity

Feature	Innate Immunity	Adaptive Immunity
Onset	Immediate	Delayed (days)
Specificity	Nonspecific	Highly specific
Memory	None	Present
Main Components	Physical/chemical barriers, phagocytes, NK cells, inflammation, fever, complement, interferons	Lymphocytes (B and T cells), antibodies
Effectiveness	General, less effective	Highly effective

Key Terms

• Phagocytosis: The process by which cells ingest and destroy foreign particles or microbes.

• Apoptosis: Programmed cell death, often induced by immune cells to eliminate infected or abnormal cells.

• Opsonization: The marking of pathogens for destruction by phagocytes.

• Cytokines: Signaling proteins released by cells to mediate and regulate immunity and inflammation.

Summary

Innate immunity provides the body's immediate, nonspecific defense against pathogens through physical and chemical barriers, defensive cells, and physiological responses such as inflammation and fever. If pathogens evade these defenses, the adaptive immune system is activated for a more targeted response. Understanding the mechanisms of innate immunity is essential for comprehending how the body initially responds to infection and injury.

Additional info: Adaptive immunity will be covered in subsequent lectures; this guide focuses on innate immunity as outlined in the provided materials.