THE CIRCULATORY SYSTEM: THE COMMON CARRIER FOR THE BODY 101 



formation of a fibrous wall around the area of infection. If the invading 

 microorganisms or the antigens are destroyed and the injured cells 

 repaired, the inflammation subsides; but if the cells in the area are killed, 

 a boil or abscess is formed — a boil if the infection is in the skin, an abscess 

 if it is in deeper tissues. Sometimes it happens that the invaders overcome 

 the localizing action of the tissues and spread until they reach the blood 

 stream. 



The defensive mechanisms of the circulatory system comprise mechanical 

 readjustments of blood and lymph flow, fixed and mobile phagocytic 

 cells, and chemical responses. We have just noted that the capillaries of 

 infected areas dilate, bringing more blood to aid the local tissue cells and 

 causing the characteristic reddening implied by the word "inflammation." 

 The lymph vessels also dilate, and tissue fluid accumulates in the inflamed 

 area. 



The phagocytes include the motile white blood cells, or leucocytes, and 

 the fixed phagocytes of spleen, liver, and lymph nodes. The leucocytes 

 are "shock troops" that seek out the invading cells. As they are brought 

 into the vicinity of the infection by the blood stream, they squeeze their 

 way out between the cells that form the thin capillary walls, and move 

 amoebalike to the infected area. There they ingest the foreign cells, and 

 in so doing generally pay with their lives. Their dead bodies, along with 

 dead tissue cells and lymph, form the pus that develops in an infected 

 wound. If the infection is small the dead cells are removed by other 

 leucocytes, but if the destruction of phagocytes and tissues is extensive 

 the pus may drain to the surface. Prolonged or extensive infection stimu- 

 lates the body to rapid production of leucocytes. The resultant high 

 "white count" of the blood may be used by the physician to diagnose 

 the existence of infection and to gain some notion of its severity. 



The fixed phagocytes that line the blood and lymph channels in the 

 spleen, liver, and lymph nodes remove foreign cells and antigen particles 

 from blood and lymph and thus filter the blood. They constitute a part 

 of the inner defense line that comes into operation when infection fails 

 to be localized and reaches the circulatory system. 



The chemical defenses of the blood are a part of the general immunity 

 reactions common to all tissues. The blood plasma becomes charged with 

 antibodies produced by cells. These antibodies include many and varied 

 substances: antitoxins that neutralize toxins; precipitins that unite with 

 antigens to form insoluble and therefore harmless precipitates; agglu- 

 tinins that cause the cells upon which they act to become clumped or 

 glued together, immobilizing and weakening or destroying them; and 

 cytolisins, which dissolve the cells against which they are specific. One 

 of the most characteristic properties of antibodies is their specificity. As 

 we have previously mentioned, each is produced in response to a particu- 



