494 IMMUNITY. 



are specially active towards bacteria, and from what can be 

 directly observed, as well as indirectly inferred, there can be no 

 doubt that such a faculty is also possessed by the phagocytes 

 of the body. Thus bacteria within these cells are in a position 

 favourable to their destruction. It is manifest that chemiotaxis, 

 which regulates the ingestion of bacteria, is a highly impor- 

 tant factor. An animal whose leucocytes are attracted by 

 the bacteria will be in a more favourable position than one 

 in which this attraction does not obtain. In the process of , 

 immunisation of a susceptible animal we see a negative or 

 neutral chemiotaxis becoming replaced by positive chemiotaxis. 

 This is, explained by Metchnikoff as due to an education or 

 stimulation of the phagocytes. It is, however, difficult to see 

 how they can be stimulated to move in a particular direction, 

 viz., towards the bacteria, and it seems more likely that in the 

 fluids of the immune animal the bacteria undergo some change 

 by which they can exert a positive chemiotaxis. This is ren- 

 dered the more Hkely by an experiment by Denys, in which 

 he showed that in a hanging-drop preparation the rabbit's 

 leucocytes behave indifferently towards pneumococci, whereas 

 on the addition of some antipneumococcic serum they moved 

 towards the pneumococci and ingested them. That the addition 

 of the corresponding immune body can change the chemiotactic 

 phenomena in this way can be readily shown in the case of red 

 corpuscles. 



The digestive ferments of phagocytes or cytases are, according 

 to Metchnikoff, retained within the cells under normal condi- 

 tions, but are set free when these cells are injured, for example, 

 when the blood is shed. They then become free in the serum 

 by the breaking up of the cells — • the process known as pha- 

 golysis — and they then constitute the alexines, or complements 

 of Ehrlich. Of these, as has already been said, he thinks there 

 are probably two kinds — one called macrocytase, contained in 

 the macrophages, which is specially active towards the formed 

 elements of the animal body, protozoa, etc. ; and the other, 

 microcytase, contained within the polymorpho-nuclear leucocytes, 

 which has a special digestive action on bacteria. It is the micro- 

 cytase which gives blood serum its bactericidal properties. 



When the properties of antibacterial sera, as above described, 

 are considered in relation to phagocytosis, Metchnikoff gives 



