284 INFECTION AND RESISTANCE 



destroying acid-fast bacilli. Indeed, much evidence tends to show 

 that the ingestion of tubercle bacilli by microphages may be a detri- 

 ment to the host, since the bacilli by this means are carried through 

 the lymphatics and variously distributed throughout the body. Poly- 

 nuclear leukocyte extracts, though containing, as we shall see, pro- 

 teolytic enzymes, do not, according to Tschernorutzky, contain any 

 lipase, and it may well be that for this reason they are unable to 

 attack the waxy substances which form an integral part of these or- 

 ganisms. This is in keeping with the observations made by Terry 

 in our laboratory, that rat leprosy bacilli may be kept within leu- 

 kocytes for weeks without losing their acid-fast properties, whereas 

 the same bacilli, as the writer and Gary found, were rapidly disin- 

 tegrated in spleen cells growing in plasma. Moreover, it is well 

 known that the estimation of tuberculo-opsonin contents of the sera 

 of tuberculous patients has been peculiarly unsatisfactory in throw- 

 ing light on the progress of the disease. It would seem, therefore, 

 that in this disease, as well as in others caused by acid-fast organ- 

 isms, the microphages play only an unimportant part in the defence 

 of the body. 



On the other hand, when tubercle bacilli are deposited either in 

 a lymphnode (through the vehicle of leukocytes) or in a capillary 

 anywhere by the blood stream, a train of cellular changes is initiated 

 in which the predominant part is played by the macrophages. The 

 tubercle bacilli so deposited are rapidly surrounded by large mono- 

 nuclear cells, probably endothelial in origin. Some of the micro- 

 organisms may even be phagocyted and taken into these cells. These 

 cells, spoken of as "epithelioid cells," surround the clump of bacteria 

 in more or less concentric rings, and around these there is an accumu- 

 lation of leukocytes, largely of the lymphocyte variety, with an ad- 

 mixture of a very few microphages. Then by the fusion of endothe- 

 lial cells, or possibly by division of the nuclei of some of these cells 

 within the individual cell bodies, giant cells are formed which take 

 up the bacilli. The further progress of the tubercle now greatly 

 depends upon the balance of power. Often such a tubercle may 

 heal, possibly because of complete intracellular digestion of the ba- 

 cilli. On the other hand growth and multiplication may lead to a 

 slow and dry necrosis of the center of such a mass of cells, leading 

 to the condition spoken of as caseation. Epithelioid cells lose their 

 outlines and staining properties, and go to pieces. The center of the 

 lesion is a grumous mass, the periphery shows a few giant cells and 

 connective tissue proliferation. 



It is always surprising to those who study these lesions for the 

 first time how rarely they succeed in finding tubercle bacilli in 

 microscopic sections prepared from such tubercles by the ordinary 

 Ziehl-Neelsen method of staining. Repeated and careful examina- 

 tion of such material may fail to reveal any acid-fast organisms, 



