590 IMMUNITY 



become destroyed. In fact, observations on phagocytosis in 

 vitro show that such destruction may in the case of some 

 organisms occur so rapidly that the actual number observable in 

 the leucocytes is no indication of the activity of the process. 

 In other instances, e.g., in gonorrhoea, the ingested organisms 

 would appear to survive a considerable time without undergoing 

 change. Undoubtedly phagocytosis is of the highest importance 

 in active immunity, as by its means organisms which would not 

 undergo an extracellular death may be killed off. In the process 

 of immunisation of a susceptible animal we see a negative or 

 neutral chemiotaxis becoming replaced by positive chemiotaxis. 

 This has been explained by Metchnikoff as due to an education 

 or stimulation of the phagocytes. The recent work on opsonins 

 shows, however, that this is not the case, as leucocytes from an 

 immunised animal are as a rule not more active in this direction 

 than those of a normal animal, the all-important factor being 

 the development of an opsonin in the immune animal. Thus this 

 phase of immunity comes to be merely an aspect of the action of 

 anti-substances in general. 



The digestive ferments of phagocytes or cytases are, according 

 to Metchnikoff, retained within the cells under normal conditions, 

 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 phagolysis 

 and they then constitute the alexins, or complements of Ehrlich. 

 Of these, as has already been said, Metchnikoff 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 

 microcytase which gives blood serum its bactericidal properties. 

 It appears to us, however, that Metchnikoff has gone too far in 

 distinguishing the activities of the two classes of cells so much 

 as he has done. 



When the properties of antibacterial sera, as above described, 

 are considered in relation to phagocytosis, Metchnikoff gives the 

 following explanation. He admits that the immune-body is 

 fixed by the bacteria (or red corpuscles, as the case may be), 

 though he does not state that a chemical combination takes 

 place ; hence he calls it a fixative (fixateur). The immune-bodies 

 are to be regarded as auxiliary ferments (ferments adjuvants} 

 which aid the action of the alexin. Unlike the latter, however, 

 they are formed in excess during immunisation and set free in 



