IMMUNITY 131 



and the result will be the forming of pseudopodia on the part of 

 the leukocytes and gradual motion in this direction. Once beyond 

 the vessel wall the leukocyte will travel in the direction from which 

 the chemotactic substance comes. If the leukocyte encounters 

 a substance that further lowers the surface tension it will encircle 

 and inclose it. If by chance it has engulfed bacteria their toxins 

 may kill the cell or so equalize the tension that it ceases to move ; 

 otherwise the leukocyte moves forward until it is checked. It 

 may be that movement continues always in the direction from 

 which the chemical stimulus comes until it is blocked by a phalange 

 of leukocytes which are being held by chemotaxis around the 

 area of infection. This supposition would explain the wall forma- 

 tion which so often occurs in suppurative processes. If, on the 

 other hand, recovery commences and the chemotactic substances 

 cease to be formed, then it may be more abundant at some dis- 

 tance away from the center, and towards this point the leukocytes 

 will move, following the attracting substance back to the lymph 

 stream and blood vessels. This probably explains the dispersion 

 of the living phagocytes at the end of an inflammatory process." 

 (Kolmer.) 



Nothing is known of the nature of these chemotactic substances. 

 It is thought that they may be formed as a result of the death of 

 tissue cells caused by bacterial poison or such irritants as coal 

 or stove dust. In most infections chemotaxis is positive; in a 

 few, however, and particularly those caused by virulent streptococci, 

 the phagocytes do not appear to be influenced by any such sub- 

 stance. Whether stimuli are ever formed by bacteria that actually 

 repel leukocytes is not as yet decided. If such substances do 

 occur they must closely resemble the aggressins which neutralize 

 opsonins and so retard phagocytosis. 



The engulfment of bacteria is accomplished by the phagocyte 

 protruding part of its protoplasm until it encircles the organism, 

 which soon appears within the substance of the phagocyte. 



After phagocytosis the fate of the inclosed bacteria depends 

 largely on their nature. Generally they undergo a process of 

 digestion similar to gastric digestion in higher animals; the 



