Inflammation and Repair: Key Concepts in Anatomy & Physiology

Inflammation and Repair

Learning Objectives

This section introduces the fundamental concepts of inflammation and tissue repair, essential for understanding the body's response to injury and infection. Students should be able to:

• Define inflammation and explain its protective role in host defense.

• Distinguish between acute and chronic inflammation.

• Describe the major causes and triggers of inflammation.

• Explain the mechanisms and key cellular events involved in inflammation.

• Identify the major chemical mediators and their functions.

• Describe the process of tissue repair following inflammation.

• Recognize the potential harmful consequences of inflammation.

Inflammation: General Properties

Definition and Function

Inflammation is the response of vascularized tissues to infections and tissue damage. It is a protective mechanism that brings cells and molecules of host defense from the circulation to the site of injury in order to eliminate offending agents.

• Purpose: Essential for survival; serves to rid the host of both the initial cause of cell injury (e.g., microbes, toxins) and the consequences of such injury (e.g., necrotic cells and tissues).

• Major mediators: Phagocytic leukocytes, antibodies, complement proteins (first responders).

Example: When a cut becomes infected, inflammation brings immune cells to the area to fight off bacteria and begin tissue repair.

Causes of Inflammation

Major Triggers

Inflammation can be triggered by a variety of factors:

• Infections: Bacterial, viral, fungal, parasitic, and microbial toxins (e.g., lipopolysaccharide).

• Tissue necrosis: Regardless of the cause, necrotic cells and tissues elicit inflammation.

• Foreign bodies: Dirt, sutures, or endogenous substances (e.g., uric acid crystals).

• Immune reactions (hypersensitivity): Normally protective immune responses can damage the individual's own tissues.

• Autoimmune diseases: The immune response is directed against self-antigens.

• Allergies: The immune response is inappropriate against environmental substances or microbes.

Example: Allergic reactions to pollen result in inflammation of the nasal mucosa.

Recognition of Offending Agents

Cellular Recognition Mechanisms

The first step in all inflammatory reactions is the recognition of offending agents by cellular receptors and circulating proteins.

• Cellular receptors: Sense the presence of foreign bodies in any cellular compartment.

• Locations:

  • Plasma membrane (for extracellular microbes)

  • Endosomes (for ingested microbes)

  • Cytosol (for intracellular microbes)

• Major receptor types:

  • Toll-like receptors (TLRs) – membrane

  • Inflammasome – cytoplasm

  • Fc receptors – membrane

  • Complement proteins – plasma

• Sentinel cells: Expressed on epithelial cells, macrophages, dendritic cells – first line of tissue exposed to foreign bodies.

Example: Macrophages in the skin detect bacteria through TLRs and initiate inflammation.

Acute vs. Chronic Inflammation

Types and Characteristics

Inflammation can be classified as acute or chronic, each with distinct features and roles in immunity.

• Acute inflammation: Vital, rapid response to infections and tissue damage; type of innate immunity.

• Onset: Develops within minutes or hours and is of short duration (hours or a few days).

• Main characteristics: Exudation of fluid and plasma proteins (edema) and emigration of leukocytes (neutrophils).

• Chronic inflammation: If the acute reaction fails to clear the stimulus, the reaction progresses for a longer period.

• Main characteristics: Involves macrophages and lymphocytes, induces angiogenesis and connective tissue growth (adaptive immunity).

Example: Persistent infection with tuberculosis bacteria leads to chronic inflammation and tissue remodeling.

Comparison Table: Acute vs. Chronic Inflammation

Feature	Acute Inflammation	Chronic Inflammation
Onset	Rapid (minutes to hours)	Slow (days to years)
Duration	Short (hours to days)	Long (weeks to years)
Main Cells	Neutrophils	Macrophages, lymphocytes
Key Features	Edema, exudation of fluid	Angiogenesis, tissue remodeling
Immunity Type	Innate	Adaptive

Additional info: Chronic inflammation is associated with many diseases, including autoimmune disorders and persistent infections, and can lead to tissue destruction and fibrosis.