Introduction
Inflammation is a complex interaction between chemical mediators and cells that occurs when tissue is injured. Inflammatory reactions are designed to eliminate the cause of the injury and repair the damaged tissue.
There are five classic signs of inflammation. Celsus, an ancient Roman philosopher, described four signs of inflammation: redness (rubor), swelling (tumor), heat (calor), and pain (dolor). Rudolf Virchow added loss of function (functio laesa) as a sequela of inflammation. Inflammatory cells (leukocytes) were first described by the Nobel laureate, Ilya Metchnikoff.Inflammation is initiated when mechanical barriers (skin and mucous membranes) are damaged by trauma or infection. Initially, inflammation is a nonspecific reaction designed to destroy the inciting agent, limit the spread of the injury, and stimulate an acquired immune response. Acquired immunity is more specific and limits repeated injury by agents via recognition of foreign antigens. Inflammation relies on interaction between damaged tissue cells, cell mediators, inflammatory leukocytes, and the blood vascular system. Note that the inflammatory process is critically linked to the blood vascular system; vascular endothelial cells regulate the inflammatory response and blood transports inflammatory mediators and cells to the site of injury. The subsets of inflammatory leukocytes present at the site of injury provide valuable information about what caused the injury, when the injury occurred (acute or chronic), and which medical treatment is most appropriate. A simplified overview of the steps involved in inflammation will be discussed; they include: (1) recognition of injury, (2) acute vascular response, (3) acute cellular response, (4) chronic cellular response, and (5) resolution. The final section of this chapter reviews important diagnostic information that is provided by cytologic evaluation of inflammatory lesions.