Inflammation and Repair: Mechanisms, Mediators, and Outcomes

Inflammation and Repair

Learning Objectives

• Define inflammation and explain its protective role in host defense against infections and tissue injury.

• Identify and describe the major causes and triggers of inflammation, including infections, tissue necrosis, foreign bodies, and immune reactions.

• Explain the mechanisms and key cellular events involved in acute inflammation, including recognition of the offending agent, vascular changes, leukocyte recruitment, and mediator release.

• Compare and contrast acute and chronic inflammation, highlighting their causes, cellular participants, and outcomes.

• List the major chemical mediators of inflammation, such as cytokines and chemokines, and describe their functions in regulating the inflammatory response.

• Describe the process of tissue repair following inflammation, including regeneration, scarring, angiogenesis, and connective tissue remodeling.

• Discuss the potential harmful consequences of inflammation, including tissue damage, chronic disease development, and systemic effects like fever and leukocytosis.

Inflammation: General Properties

Definition and Protective Role

Inflammation is the response of vascularized tissues to infections and damaged tissues. It brings cells and molecules of host defense from the circulation to the site of injury in order to eliminate the offending agents.

• It is a protective response essential for survival.

• Serves to rid the host of both the initial cause of injury (e.g., microbes, toxins) and the consequences of such injury (e.g., necrotic cells and tissues).

• Major mediators: phagocytic leukocytes, antibodies, and complement proteins—these are the first responders.

Causes of Inflammation

Major Triggers

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

• Tissue necrosis: Regardless of the cause.

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

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

• If the immune response is directed against self-antigens: autoimmune diseases.

• If the response is inappropriate against environmental substances/microbes: allergies.

Recognition of Offending Agents

Cellular and Molecular 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 specific cellular compartments:

  • 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

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

Acute vs. Chronic Inflammation

Comparison of Types

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

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

• Characteristics: Exudation of fluid and plasma proteins (edema) and emigration of leukocytes (mainly neutrophils).

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

• Chronic inflammation recruits macrophages and lymphocytes, induces angiogenesis and connective tissue growth (adaptive immunity).

Acute Inflammation

Major Components

• Dilation of small vessels and an increase in blood flow.

• Increased permeability of the microvasculature to allow plasma proteins and leukocytes to leave the circulation.

• Migration of leukocytes from the microcirculation to the site of injury, and their activation to eliminate the offending agent.

Sequence of Acute Inflammation

Stepwise Process

1. Causes of inflammation

2. Recognition of the offending agent

3. Acute inflammation

4. Phagocytosis and clearance of offending agent

Additional info: Expanded Academic Context

• Exudation refers to the movement of fluid, proteins, and cells from the vascular system into the interstitial tissue or body cavities, typically seen in inflammation.

• Edema is the accumulation of excess fluid in tissues, often a result of increased vascular permeability during inflammation.

• Phagocytosis is the process by which cells (mainly neutrophils and macrophages) engulf and digest microorganisms and cellular debris.

• Angiogenesis is the formation of new blood vessels, which is important for tissue repair and regeneration.

• Adaptive immunity involves lymphocytes (T and B cells) and develops more slowly but provides long-lasting protection.