Inflammation: Causes and Purpose

Inflammation

• Inflammation is a protective response aimed at eliminating the initial cause of cell injury, as well as necrotic cells and tissues resulting from the original injury.

Causes of Inflammation

• Exogenous causes: trauma, surgery, infection, caustic chemicals, extremes of heat or cold, immune responses
• Endogenous causes: ischemic damage

Purpose of Inflammation

• Destroy invading microbial agents
• Contain and isolate (limit) injurious agents
• Inactivate toxins
• Remove necrotic tissue to allow healing and repair

Generalities about Inflammation

• Inflammation is a process involving multiple cellular, humoral, and tissue participants
• Inflammation occurs only in living tissue, cannot occur in dead tissue
• Inflammation is a series of events that form a continuum, a defensive reaction that always requires an irritating stimulus

Types of Inflammation

• Acute inflammation: rapid in onset, short duration, characterized by exudation of fluid and plasma proteins, and emigration of leukocytes, predominantly neutrophils
• Chronic inflammation: longer duration, associated with the 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, and little exudation
• Moderate reactions: some damage to host tissue, easily visible host reaction to the injury, 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
• In between lies a variable span of time in which the character of the evolving reaction can change

Inflammatory Cells

• Neutrophils: first leukocytes to gather at sites of acute inflammation, constitute the cellular defense system against bacteria, participate in phagocytosis, and release lytic lysosomal enzymes
• Eosinophils: abundant at sites of inflammation in diseases of immunologic or allergic origin, respond to antigen-antibody complexes
• Basophils and mast cells: release heparin and histamine in response to antigen-antibody complexes
• Monocytes and macrophages: originate from circulating monocytes, larger than neutrophils, possess gray-blue cytoplasm, participate in phagocytosis, and contain phagocytic inclusions

Signs and Symptoms of Inflammation

• Heat (increase in 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 movements or function due to pain and tissue damage

Components of Inflammation

• Alterations in vascular caliber that lead to an increase in 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

Acute Inflammation Sequence of Events

• Vasodilation
• Increased vascular permeability
• Leakage of exudate
• Margination, rolling, and 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, induced by the action of several mediators, notably histamine and nitric oxide

Chemical Mediators of Acute Inflammation

• Histamine: released from mast cells, increases blood flow to the area, and leakage of fluid and proteins from the blood into the tissue space
• Nitric oxide: produced by endothelial cells and macrophages, causes smooth muscle relaxation in the vessel wall, and reduces platelet activation and aggregation