248 BACTERIA 



He came to the conclusion that the successful resistance 

 which an animal offered to bacteria depended upon the 

 activity of these scavenging cells, or phagocytes. These 

 cells are derived from various cellular elements normally 

 present in the body : leucocytes, endolethial cells, connect- 

 ive-tissue corpuscles, and any and all cells in the body which 

 possess the power of ingesting bacteria. If they are present 

 in large numbers and active, the animal is insusceptible to 

 certain diseases; if they are few and inactive, the animal is 

 susceptible. 



It appears that the bacteria or other foreign bodies in the 

 blood which are attacked by the phagocyte become assimi- 

 lated until they are a part of the phagocyte itself. Met- 

 schnikoff explained also how it comes to pass that the 

 phagocyte is able to encounter bacteria when both are cir- 

 culating through the blood. It is guided in this attack 

 upon the organisms by a power termed chcmiotaxis. The 

 bacteria elaborate a chemical substance which attracts the 

 phagocyte, and this is termed ** positive chemiotaxis. '* ' 

 But it may occur that the chemical substance produced by 

 the bacteria may have an opposite, or repellent, effect upon 

 the leucocytes, in which case we have ** negative chemio- 

 taxis. " " It is not to be wondered at that such a theory of 

 immunity based upon microscopical observations, should at 

 first have been widely accepted, and there can be no doubt 

 that Metschnikoff has collected a considerable mass of evi- 

 dence in support of a theory of phagocytosis. But when it 

 came to be known that blood serum, from which all leuco- 

 cytes (phagocytes) had been removed, possessed the same 

 immunising effect as before, it was clear that such effect was 

 a property of the serum per se, and not only or wholly due 

 to the scavenging power of certain cells in it. Even the 



' Types of bodies possessing positive chemiotaxis for bacteria are the salts 

 of potassium, peptone, glycerine. 



^ Negative chemiotaxis is illustrated in alcohol, and free acids, and alkalies. 



