Acute and Chronic Inflammation

Inflammation Characteristics

• Acute inflammation: rapid in onset (seconds or minutes), short duration (minutes, hours, or few days), characterized by exudation of fluid and plasma proteins (edema), and emigration of leukocytes (predominantly neutrophils)
• Chronic inflammation: longer duration, associated with presence of lymphocytes and macrophages, proliferation of blood vessels, fibrosis, and tissue injury

Degree of Severity

• Mild reactions: little or no tissue destruction, slight evidence of vascular involvement (hyperemia and edema), little exudation
• Moderate reactions: some damage to host tissue, visible host reaction to injury, manifest by leukocytic accumulation and vascular phenomena
• Severe reactions: considerable tissue damage, abundant exudation

Duration of Inflammatory Reactions

• All inflammatory reactions have a beginning and most have an end
• Character of the evolving reaction can change over time

Per Acute Inflammation

• Manifests very soon after initiation (only a few hours)
• Caused by a potent stimulus
• Slight edema, hyperemia, hemorrhage, and few leukocytes beginning to infiltrate the damaged tissue

Cells of Inflammatory Exudates

• Three basic types of white blood cells involved in inflammatory reaction: polymorphonuclear leukocytes (granulocytes), eosinophils, and basophils

Neutrophils

• First leukocytes to gather at sites of acute inflammation
• Cells of the cellular defense system against bacteria
• Major functions:
  • Participation in phagocytosis
  • Ingest, neutralize, and destroy ingested particles
  • Release of lytic lysosomal enzymes
  • Formation of chemotactic factors

Eosinophils

• Abundant at sites of inflammation in diseases of immunologic or allergic origin
• Prominent in reactions caused by parasites
• Eosinophil-specific chemotaxis has been defined
• Responsive to antigen-antibody complexes

Basophils and Mast Cells

• Both cells release heparin and histamine in response to antigen-antibody complexes

Monocytes and Macrophages

• Originate from circulating monocytes of bone marrow origin
• Larger than neutrophils
• Possess gray-blue cytoplasm filled with very fine granules
• Major function: phagocytosis
• Contain phagocytic inclusions (phagosomes) that enclose bacteria, cell debris, and lipid residues from dead cell membranes
• Source of multinucleated giant cells present in some chronic inflammatory reactions
• Another type of macrophage seen in some chronic reactions: epithelioid cell
• Large, pale-staining macrophages with ovoid nucleus and angular shape resembling epithelial cells

Lymphocytes and Plasma Cells

• Both involved in the immune response
• Cellular mediators of both immediate antibody response and delayed cellular hypersensitivity response
• Lymphocytes originate in lymphoid tissue, such as lymph nodes, spleen
• Plasma cells originate from lymphocytes committed to antibody production

Types of Inflammation

• Inflammation is divided into acute and chronic patterns

Signs and Symptoms of Inflammation

• Heat (increased temperature) caused by increased blood flow
• Pain caused by irritation of nerve endings by chemical mediators
• Swelling of tissue caused by vascular dilatation and accumulation of inflammatory fluid and cellular exudate
• Redness of tissue caused by vascular dilatation
• Loss of movement or function due to pain and tissue damage

Components of Inflammation

• Alterations in vascular caliber that lead to increased blood flow (vasodilatation)
• Structural changes in the microvasculature that permit plasma proteins and leukocytes to leave the circulation (increased vascular permeability)
• Emigration of leukocytes from the microcirculation, their accumulation in the focus of injury, and their activation to eliminate the offending agent (cellular recruitment and activation)

Acute Inflammation Sequence of Events

• Vasodilation
• Increased vascular permeability
• Leakage of exudate
• Margination, rolling, adhesion
• Chemotaxis
• Phagocytosis

Vascular Changes

• Changes in vascular flow and caliber begin early after injury and develop at varying rates depending on the severity of the injury
• Vasodilation is one of the earliest manifestations of acute inflammation
• Vasodilation first involves arterioles and then results in opening of new capillary beds in the area

Chemical Mediators of Acute Inflammation

• Chemical mediators are chemical factors derived from plasma and cells, including neutrophils, monocytes, macrophages, mast cells, endothelium, smooth muscles, fibroblasts, platelets, and damaged tissue cells
• Chemical mediators act by binding to specific receptors on target cells

Histamine

• Released from mast cells
• Increases blood flow to the area and leakage of fluid and proteins from the blood into the tissue space
• Produces redness and swelling associated with inflammation

Nitric Oxide

• Produced by endothelial cells and macrophages
• Causes smooth muscle relaxation in the vessel wall (vasodilation) and reduces platelet activation and aggregation