254 ACTION OF " CYTASE IN PHAGOCYTOSIS 



injurious substance is provided by the polynuclears, which thus 

 play both parts. 



Kanthack's experiment is the best and most direct evidence of 

 the extracellular injury of bacteria by substances derived from 

 the leucocytes occurring as a preparation for phagocytosis. 



Metchnikoff resisted these views for a time, but soon had to 

 admit that phagocytosis is not the only factor in immunity ; and 

 he then altered his theory in an ingenious way, and regarded the 

 extracellular injury or solution of organisms as being essentially 

 the same process as that by which they are digested after being 

 taken in by the phagocytes. He considers that bacteria, red 

 corpuscles, etc., after being taken in, are digested by the action 

 of a proteolytic enzyme which he calls " cytase " a term which 

 has been already alluded to as a synonym for complement or 

 alexin. Of this there are two sorts : macrocytase, which is 

 formed by the macrophages, and which digests corpuscles, cells, 

 etc. ; and microcytase, formed by the polynuclears, and powerful 

 against bacteria. Ordinarily these enzymes are restricted to the 

 cells which form them, and where ingested bodies are contained 

 in vacuoles, these latter contain a solution of the suitable cytase ; 

 but when solution of the phagocytes occurs the cytase is set at 

 liberty, and may then exert its action on cells or bacteria which 

 are lying free. Metchnikoff regards this as a process of much less 

 importance than phagocytosis, and points out that the solution 

 which it brings about is rarely complete : thus, when bird's 

 corpuscles are ingested, they are entirely absorbed, nuclei and 

 all ; whereas when they are acted on by a haemolysin (which 

 Metchnikoff regards as a macrocytase), the nuclei remain. This 

 is certainly true as regards the action of most sera on bacteria, 

 solution being rarely complete, and it is only in the highly potent 

 sera obtained by prolonged immunization to certain bacteria that 

 complete disappearance of the bacteria occurs as a result of the 

 action of serum ; yet when taken up by the leucocytes they are 

 digested altogether, sometimes with great rapidity. 



The difference between these views and those of the cellulo- 

 humoralists is roughly this : Metchnikoff looks upon the protective 

 substance as a digestive enzyme which has for its object the 

 transformation of the foreign cells, etc., into proteids suitable for 

 the nourishment o the phagocyte ; whereas most bacteriologists 

 regard them as being allied to the toxins rather than to the 

 enzymes, and as being specially intended for the defence of the 



