PREPARATION OF SERUM-FREE LEUKOCYTES 59 



If, for instance, such an attenuated strain of the anthrax bacillus 

 (anthrax vaccine) is injected into a susceptible animal, its leukocytes, 

 heretofore non-phagocytic with. reference to anthrax bacilli, are en- 

 dowed with phagocytic properties. 



Stages. There are distinguished in phagocytosis three different 

 stages namely, the approach of the phagocyte to the bacterium; 

 inclusion, and finally, digestion, or destruction, of the microorganisms. 



Chemotaxis. It is one of the most remarkable facts that phago- 

 cytes possess a bacteriochemical sensibility. This term means that 

 phagocytes are attracted toward certain bacteria, as iron filings are 

 attracted to a magnet. It is easy to demonstrate this and to show 

 how phagocytes can even wander through a vessel wall toward a 

 place where certain bacteria have been injected. This peculiar 

 attraction which acts over a distance is known as a positive chemo- 

 taxis or chemiotaxis. It can, on the other hand, be shown that some 

 bacteria have a repelling influence toward phagocytes which causes 

 them to flee. This repelling power is called negative chemotaxis. 

 When bacteria exert neither an attracting nor a repelling influence 

 upon polynuclear leukocytes, we speak of an indifferent chemotaxis. 



Aggressins. It is a very interesting fact that the most virulent 

 strains of certain bacteria, for instance, very virulent streptococci or 

 pneumococci, exert a repelling influence toward the wandering leuko- 

 cytes, and in this manner prevent phagocytosis. It is very prob- 

 able that the power of such virulent strains of pathogenic bacteria 

 depends upon certain of their secretory products, and these have been 

 called aggressins. 



Phagocytosis and Spreading of Disease. In considering the process 

 of phagocytosis as one of the protecting agencies of the body against 

 the invasion and multiplication of pathogenic bacteria, it is impor- 

 tant and necessary to remember that phagocytes often swallow more 

 bacteria than they can digest. These bacteria remain alive, secrete 

 their toxins, weaken, and finally kill the leukocytes. Death of the 

 latter may not occur for some time, and meanwhile they may wander 

 away after having engulfed several bacteria, and thus transport infect- 

 ing bacteria to a place distant from their first point of entrance. In 

 other words, under some conditions phagocytes, instead of acting for 

 good, as is generally the case, may do harm by spreading infection 

 from one place to another. 



Preparation of Serum-free Leukocytes. Leukocytes of man or 

 animals can be prepared in such a manner that they are free from 

 every trace of blood serum. This is done in the following manner: 

 About fifteen to twenty drops of blood are drawn by puncturing a 

 finger or an ear with a sterile surgical needle and the blood is allowed 

 to fall directly into a glass centrifuge tube which contains a 1^ per 

 cent, watery solution of sodium citrate. This solution prevents the 

 coagulation of blood. Blood and citrate solution are mixed by shaking 

 and the mixture is then centrifuged until a clear upper stratum is 



