ON IMMUNITY TO BACTERIA 343 



containing the bacteria are often singularly free from leucocytes, 

 be the explanation what it may. 



The first necessity for virulence, therefore, is the possession of 

 an active toxin, which, either by lowering the general vitality, 

 or by repelling, paralyzing, injuring, or killing the leucocytes, 

 diminishes the amount of phagocytosis which occurs. 



A second defence against phagocytosis is the production of a 

 defensive envelope. This is well seen in the case of virulent 

 anthrax bacilli, which form thick gelatinous envelopes in the 

 animal body. Such capsulated forms are only taken up with 

 difficulty by the leucocytes, and the greater the power of capsule 

 formation the greater the virulence of the culture. Similar 

 phenomena have been seen in the case of the streptococci, 

 tubercle bacilli, and other organisms. The capsule of the 

 pneumococcus, which is only developed in the animal body or in 

 culture fluids approximating thereto, is probably a case in point. 



Capsule formation is also probably a defence against bacterio- 

 lysins. Thus, according to Eisenberg, virulent typhoid or coli 

 bacilli stain blue by Giemsa's method, and show a pink edge. 

 Organisms which have undergone this modification are suscep- 

 tible to phagocytosis, but show great resistance to bacteriolysis. 

 And it is probable that the capsule of the anthrax bacillus is 

 intended largely as a defence against bacteriolytic substances. The 

 bacilli can be trained to produce it by cultivation in the serum of 

 the rat, which dissolves the unaltered bacilli in large numbers. 



A third and more subtle method in which a bacterium can 

 increase its virulence is by loss of the receptors (which it possesses 

 in the avirulent state), which have the power of anchoring the 

 defensive substances of the blood. Thus the virulent pneumo- 

 cocci which (as shown by Rosenau) are not ingested by leucocytes 

 in presence of serum escape, as a result of the fact that they do 

 not absorb opsonin. Typhoid bacilli which have been cultivated 

 in immune serum lose their power of combining with amboceptor, 

 and also with agglutinin, and it may be taken as a general rule 

 that the more virulent a culture the less its reaction to its agglu- 

 tinin, and the less it absorbs that substance. Thus typhoid bacilli, 

 when isolated from the body, are usually refractory to agglutina- 

 tion, and gain the property only after several generations of culture 

 on artificial media. 



Hence, therefore, we may picture the process which goes on 

 when a dose of pathogenic bacteria gains access to the blood of a 



