146 INFECTION AND IMMUNITY. 



kind of leucocyte, that others attract another 

 kind, and that in still other instances the leuco- 

 cytes appear to be either uninfluenced or actually 

 are repelled by the infecting agent. 



The phenomenon of living cells moving toward 

 or away from certain other cells or substances .is 

 an expression of affinity and this affinity is known 

 as chemotaxis; the former is positive, the lat- 

 ter negative, chemotaxis. There is a somewhat 

 general law, but one to which exceptions exist, 

 that, regardless of the microbe involved, the more 

 acute the inflammatory process the more do poly- 

 morphonuclear leucocytes accumulate, while in 

 the more chronic infections, with much connec- 

 tive tissue formation, the mononuclear leucocytes 

 predominate. Thus in tuberculosis one finds 

 lymphocytes and plasma cells mononuclears 

 predominating greatly over the polymorphonu- 

 clears. In the acute purulent infections 1 , on the 

 other hand streptococcus, staphylococcus, pneu- 

 mococcus the latter type of leucocyte predomi- 

 nates, the mononuclears being fewer and remain- 

 ing at a distance from the center of action. There 

 is reason to believe that the mononuclear leuco- 

 cytes play an important, though perhaps indirect, 

 role in the formation of the connective tissue. 



The ingestion of particles by living cells, phago- 

 cytosis, is a property which many cells possess. 

 Although micro-organisms and inanimate parti- 

 cles are sometimes found in epithelial cells, cer- 

 tain of the mesoblastic cells have this function 

 pre-eminently : polymorphonuclear leucocytes, large 

 mononuclear leucocytes (lymphocytes), ameboid 

 connective tissue and endothelial cells. Of these 

 the polymorphonuclear leucocytes, the microphages 



