248 SUSCEPTIBILITY AND IMMUNITY. 



intraceUular. Many of the facts bearing upon the terms of this relationship 

 between tissue cell and microorganism are now well known, yet it is worth 

 while to recapitulate the more important, in order to show that from them it 

 is possible to gain a general law ; and what is more, that from a study of 

 such facts some insight may be gained into the phenomena of immunity. 



It may, in the first place, be postulated that whenever a microorganism 

 is discoverable within a cell its passage thither has been by means of proto- 

 plasmic or amoeboid movements, either on the part of the microbe or of the 

 cell itself. The first alternative is the rarer, although it certainly exists, and 

 of this the malarial parasite affords an excellent example; for here in the 

 amoeboid stage of its existence the hsematozoon makes its way into the in- 

 terior of a cell that possesses no active movements of its own, namely, the 

 red blood corpuscle, and from the substance of this corpuscle the parasite 

 gains its nourishment. Other sporozoa furnish instances almost equally 



good. Moz-e commonly, however, as in the case of all bacteria, where we 

 ave to deal with microorganisms which, even when mobile, are destitute of 

 protoplasmic appendages, it is the cells which play the active part ; certain 

 cells include the parasites. Of such the amcebiform leucocyte of the blood 

 and lymph is the most typical example, capable, as it is, of sending out 

 pseudopodia in all directions, while a closely allied form is the cell of the 

 splenic pulp. But there are also cells as, for instance, those forming the 

 endothelial lining of the vessels which are very definitely fixed, which 

 nevertheless can give off protoplasmic processes from their free surface and 

 so capture and include bacteria. 



All these may be spoken of as phagocytes, and may be divided into the 

 two broad groups of fixed phagocytes (endothelial cells, etc.) and free (leu- 

 cocytes). Not that the terms "phagocyte" and "leucocyte" are synonymous, 

 for of the latter three main forms may be distinguished, of which one is 

 practically immobile and never takes up bacteria. This is the lymphocyte, 

 characterized by its relatively small size, its large single nucleus, and the 

 small amount of surrounding protoplasm. The two remaining (phagocytic) 

 forms are, first, the large uninuclear leucocyte, whose prominent nucleus is 

 at times lobed or reniform, which stains well with aniline dyes and possesses 

 much protoplasm and active amoeboid movements the macrophage and, 

 second, the microphage, a small form, also staining well, but either multi- 

 nuclear or with one nucleus in the process of breaking up. If 'now we com- 

 pare the endothelial cells with these, it is evident that their properties con- 

 nect them closely with the macrophage ; and, in fact, there is now little or 

 no doubt that a very large proportion of the macrophages are of endothelial 

 origin. 



Leaving aside the subject of amoeboid microbes and their life within ani- 

 mal cells, it is to the phagocytes and their relation to the bacteria that I wish 

 specially to draw your attention. 



Taking as wide a view as possible of this relationship, we can first deter- 

 mine that the more malignant the microorganism the rarer is its presence 

 within the phagocyte. Thus in those which of all diseases are the most 

 rapidly fatal in chicken cholera affecting birds and rabbits, in hog cholera 

 ("cholera des pores") given to pigeons and rabbits, in the anthrax of mice 

 and other specially sensitive animals, in the "septicemie vibrionienne" of 

 guinea-pigs and birds, and in yet other diseases of peculiarly swift course 

 the corresponding bacteria are only very exceptionally to be found within 

 the cells, but remain free in the neighborhood of their introduction arid 

 thence invade the blood. For all the above-mentioned diseases are not 

 localized, but, on the contrary, present the characters of general acute sep- 

 ticaemia, causing death within twenty to thirty-six hours, or, in certain 

 cases, even within six hours. 



And when we pass to those diseases in which the bacteria are to be found 

 either in part or almost wholly within the phagocytes, the same law still 

 applies ; for in such cases the disease has lost its suddenness, tending to 

 have a slower course, or, indeed, to be of a chronic nature. Even in those 



