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Introduction to histopathology
Introduction to histopathology

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1.2 Acute inflammation

Inflammation is a common response to tissue injury or infection. Acute inflammation develops quickly and resolves within days, whereas chronic inflammation can last for months or years, usually because of the persistence of the initiating factor. The histological appearance of acute inflammation is quite different from chronic inflammation and the distinctive features can point to the initiating agent. For example, an infection of the skin with Staphylococcus aureus usually produces an acute inflammatory response, whereas infection with Mycobacterium leprae (leprosy) typically produces persistent infection and chronic inflammation.

There are three main components of inflammation (Figure 4):

  1. An increase in the blood supply to the affected area, caused by dilation of arterioles supplying the area.
  2. An increase in the permeability of capillaries, which allows larger serum molecules such as antibodies to enter the tissue.
  3. Migration of leukocytes from the blood into the tissues - the cells cross the endothelial cells, which line the venules, and then move out into the tissue. This process is mediated by signalling molecules called chemokines, which are bound to the endothelial surface.
Figure 4 The three main features of inflammation are controlled at different places in the vasculature. The diagram shows a longitudinal section through an arteriole, capillary and venule. Smooth muscle in the arterioles controls blood flow into the site of inflammation. Exudation of serum molecules occurs in capillaries as endothelial cells retract in response to inflammatory mediators. This allows antibodies and molecules of the complement system to enter the site of inflammation. Migration of leukocytes takes place in venules, partly because the shear force is lowest in venules and partly because signalling molecules are present on the endothelium of the venules which attract leukocytes at this point.

All of these processes bring the defence systems of the body to the affected area. The blood contains a number of proteins that stop bleeding, help clear infection and induce repair or regeneration of the tissues. It also contains different types of leukocyte (white blood cells), each of which has evolved to deal with different types of infection. One of the key histological differences between acute and chronic inflammation is seen in the sets of leukocytes that are present in the tissues. In acute inflammation polymorphonuclear neutrophils usually predominate, whereas macrophages and lymphocytes predominate in chronic inflammation. Eosinophils are often prevalent in sites of helminth infections. Hence the characteristics of inflammation are determined both by the tissue in which it occurs and by the initiating agent and its persistence.