combination of vasodilation with thickening of the blood (due to fluid leaking out of the vessels) causes a slowing of flow rate, which encourages leukocytes (white blood cells; leuko = white, cyte = cell) to stick to the sides of the vessels. This is called "margination" or "pavementing" (the white blood cells gather along the endothelium, like bricks paving a road). (Example from WebPath.)
- From here the leukocytes crawl between the endothelial cells and enter the inflamed connective tissue. Increased metabolic activity associated with leukocyte activity also generates heat (calor), contributing to local warmth.
Pain and/or itching (dolor) is caused by direct action on nerve endings of the chemical agents released during inflammation.
INFLAMMATORY INFILTRATE (Introduction to inflammation)
The inflammatory or leukocytic infiltrate consists of white blood cells which leave the blood and enter (infiltrate) the inflamed connective tissue
- Images of inflammatory infiltrate in skin.
- Image from WebPath of inflammatory exudate in lung alveoli.
Cells of the inflammatory infiltrate include neutrophils, lymphocytes and monocytes. Immigration of these cells into peripheral tissues is one of the principal purposes for inflammation, bringing to a site of injury the immune-system cells which can combat infection and clean up damaged tissue.
Neutrophils (neutrophilic leukocytes) are the first white blood cells to enter the tissue during acute inflammation. Neutrophils are anti-bacterial cells which lyse (break down) bacterial cells by releasing lysosomal enzymes. Neutrophils recognize bacteria as foreign by the antibody molecules which have attached to the bacterial surface. Antibody molecules (molecules which bind to one specific antigen or foreign substance which the body has previously encountered) are found in blood plasma and interstitial fluid.
Neutrophils are the most numerous of the leukocytes, about 60% of the white blood cell count. They are about 12 Ám in diameter in blood smear preparations (about twice the size of red blood cells), and are polymorphonuclear (meaning their nuclei have a variable shape with several lobes; neutrophils are also called PMNs or polys, short for polymorphonuclear neutrophilic leukocytes). The cytoplasm contains many lysosomal granules (vesicles containing stored lysosomal enzymes) whose specific staining properties give these cells their name. The granules are neutrophilic, meaning they do not show a special affinity for either acidic or basic stains, but are stained mildly by both. This is in contrast to the specific granules of eosinophils, which stain red with eosin, and basophils, which stain with basic stains. Severe inflammation may increase the numbers of neutrophils in blood, resulting in neutrophilia (blood smear image from WebPath). To view an electron micrograph, see WebPath.
Neutrophils are only occasionally seen in tissue sections outside blood (except, of course, in inflamed tissue). Here they may be most easily recognized by their lobed nuclei. One neutrophil nucleus might be mistaken for a cluster of very small nuclei, but each of the lobes is much smaller than any whole nucleus--only two or three Ám across, much smaller than the nuclei of lymphocytes which are among the smallest cells.
[More on blood cells.]
Lymphocytes accumulate somewhat later during the inflammatory process. Their presence in large numbers indicates the continuing presence of antigen and thus may suggest an established infection. Lymphocytes produce the multitude of diverse antibody molecules (one specific type of antibody per lymphocyte) which provide the mechanism for chemical recognition of foreign materials (distinguishing between self and non-self) and so for mediating and regulating immune responses.
Lymphocytes travel in the blood, but they routinely leave capillaries and wander through connective tissue. Therefore, lymphocytes may be normally encountered at any time in any location. They even enter epithelial tissue, crawling between the epithelial cells. They reenter circulation via lymphatic sys