128 IMMUNIZATION OF LEUCOCYTES 



under identical conditions. Subsequently, as we have seen above, 

 some slight indications of a difference have been found. But on 

 MetchnikofFs theories there ought to be a very marked difference, 

 and we should expect that, the leucocyte being par excellence the 

 cell devoted to the protection of the animal body against infections, 

 it would be the first to acquire increased resistance in acquired 

 immunity, and we should expect, e.g., leucocytes from a convales- 

 cent case of pneumonia or from an animal vaccinated against the 

 pneumococcus to take up far more pneumococci than leucocytes 

 from a normal person under similar conditions, whereas the 

 difference, if one exists, is extremely slight. A careful considera- 

 tion of the conditions of opsonic experiments leads to the con- 

 clusion that the results obtained are not to be regarded in any 

 sense as an index of the immunity of the leucocytes. Ledingham 

 has shown that the number of bacteria taken up by the leucocytes 

 depends on the extent to which the former have been altered by 

 the action of serum, and not on the temperature at which the 

 phagocytosis takes place. Thus, if the bacteria are sensitized by 

 serum, mixed with leucocytes, and part kept at 18 C. and part at 

 37 C., the number of leucocytes is the same in the two cases. 

 Now at the lower temperature no active movements of the 

 leucocytes take place, so that we cannot regard the ingestion of 

 the bacteria as being entirely, as was formerly assumed, an active 

 process brought about by a seizure or surrounding of the 

 organism by pseudopodia, as can be seen so readily in the 

 amoeba. Such a process does occur to some extent, and can be 

 seen to occur under suitable conditions ; but it is also true that 

 bacteria can be seen to make their way into a cell which has 

 been watched continuously under a high power of the microscope, 

 and in which no movement of any sort has been witnessed. We 

 are led, therefore, to believe that the phagocytosis which occurs 

 in opsonin experiments in vitro is a process allied to agglutination 

 rather than to an actual physical seizing of an organism. It is 

 one which can take place without an actual conscious if we may 

 use the term movement of the leucocyte towards its prey. 

 An immunized leucocyte would be immune to bacteria which it 

 had ingested, to the endotoxins liberated in the process of 

 bacteriolysis, whether taking place within the leucocyte or outside 

 it, and to endotoxins ; and it would exert its physiological functions 

 of movement (pseudopodia- production and chemotaxis) equally 

 well whether these toxins were present or absent. Or the effect 



