IMMUNOLOGY - Roitt A. - Mir 2000

Chapter 1. Overview of the Immune System

INFLAMMATION

Normally, immune system Cells are dispersed throughout all body Tissues, but when an infection occurs, these cells and their secreted products concentrate precisely at the site of infection. The process responsible for this is called the inflammatory response. Inflammation is characterized by three main features:

✵ Blood supply to the infected area increases;

✵ due to the contraction of endothelial cells, the permeability of blood capillaries rises; As a result, larger molecules than usual can escape from the capillaries, allowing SOLUBLE IMMUNE MEDIATORS to reach the site of infection;

✵ leukocytes migrate from venules into the surrounding tissues. In the very Cytology/cytology/16.html">Early stages of inflammation, neutrophils predominate at the infection site, but monocytes and lymphocytes migrate there later.

Cell migration and chemotaxis. A key moment in cell migration is their adhesion (spreading) to the vascular endothelium of inflamed tissues as a result of the interaction between specific molecules On the surface of leukocytes and activated endothelial cells (Fig. 1.16). Having penetrated into the tissues, cells migrate toward the site of infection under METABOLISM/18.html">The Influence of chemical attraction known as chemotaxis.

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Fig. 1.16. This scanning electron micrograph shows leukocytes adhering to the wall of a venule in inflamed tissue. (x 16,000. Photo kindly provided by Prof. M. J. Karnovsky.)

Phagocytes possess the property of actively migrating along the concentration gradient of specific (chemotactic) compounds. Particularly strong chemotaxis is induced by a fragment of one of the Complement components, C5a (Fig. 1.17), which attracts neutrophils and monocytes. When a preparation of purified C5a is applied to the Skin in vivo, neutrophil adhesion to the endothelium of nearby venules can soon be observed. Slipping then between the endothelial cells, neutrophils penetrate through the basement membrane of the venules into the tissue. This process, termed diapedesis, is described in detail in Chapter 5.

Fig. 1.17. An infectious agent causes tissue damage and complement activation in the area of inflammation. This, in turn, leads to the release of inflammatory mediators (for example, C5a, one of the most important chemotactic Peptides—a fragment of the fifth complement component). Inflammatory mediators diffuse toward nearby venules, where they cause phagocytes to adhere to the endothelium. The adherent phagocytes extend their pseudopodia between the endothelial cells and dissolve the basement membrane. They then leave the Blood Vessels and move along the concentration gradient of chemotactic mediators toward the site of inflammation (chemotaxis).