134 



SCIENCE 



"N. S. Vol. XXVII. No. 



the body, strictly speaking ; but foci of de- 

 velopment may exist in the kidney for 

 many months, infecting- the urinary blad- 

 der, and in bone and muscle, and they are 

 strictly within the body. A distinction is 

 not readily made between capacity of 

 growth within and on the surface of the 

 body, but evidence exists tending to show 

 that certain tissues may develop immunity 

 to pathogenic bacteria which usually in- 

 jure them, and certain bacteria develop 

 capacity to survive under conditions which 

 are usually fatal to them. 



It is just in this connection that we are 

 learning that bacteriolysis and bactericidal 

 effects do not necessarily go along with 

 spontaneous recovery from and acquired 

 immunity to bacterial diseases. These 

 forces of immunity may be in active opera- 

 tion, so far as tests made outside the body 

 with the blood indicate, at a time that the 

 very bacteria from and against which they 

 have developed may still be surviving in 

 the body. Typhoid bacilli have been culti- 

 vated from the blood long after the sub- 

 sidence of symptoms of typhoid fever and 

 at a time when the titre of serum bacteri- 

 olysis was of prodigious height; pneu- 

 mococci have been detected in the circula- 

 ting blood of animals actively immunized to 

 the pnerunococcus ; anthrax bacilli have 

 been grown from the blood of immune and 

 healthy sheep protected by anthrax vac- 

 cine, and living virulent tubercle bacilli of 

 the human type have been obtained from 

 the healthy lymphatic glands of calves 

 inoculated with Bovo-vaccine and in con- 

 sequence already immune to bovine tuber- 

 culosis. It is clear, therefore, that the im- 

 mune state, so far as bacteria are con- 

 cerned, can be no one-sided phenomenon in 

 which the fact of all importance is the con- 

 dition of the host, and that of small im- 

 portance the condition of the invading 

 bacterium. The phenomenon is, indeed, a 

 reciprocal one and must take account of a 



high degree of capacity for adaptive 

 changes on the part of the parasite as well 

 as on the part of the host. 



Many of the diseases due to protozoa 

 show in a more striking way the same facts 

 of mutual adaptation of parasite to host; 

 and this power of survival of the parasites 

 in the healthy body is what makes suppres- 

 sion of diseases transmitted from host to 

 host by blood-sucking insects a matter of 

 such difficult and uncertain achievement. 

 The fact as regards malaria is well known, 

 and this paradoxical condition of immunity 

 and infection is established as true for 

 trypanosomic, piroplasmic and spirillar 

 diseases of man and animals. Koch has 

 drawn attention, in a recent report, to the 

 existence in the blood of healthy natives 

 of the trypanosoma of sleeping-sickness 

 from which the Glossina may readily be- 

 come infected and made able to carry the 

 disease. It is clear that the future studies 

 in immunity will take into more direct ac- 

 count the changes in the infecting parasite 

 produced by the immune state, and will 

 seek means of their suppression which will 

 leave the host uninjured and weaponed for 

 a more successful resistance to invasion. 

 In this field prediction is hazardous; but 

 it need not excite great surprise if this de- 

 sideratum should be accomplished through 

 specific drugs suited to the purpose by 

 subtile molecular adaptations, rather than 

 by sera prepared by immunization with the 

 parasites themselves. 



The long discussion of the part played in 

 natural and acquired immunity by the 

 blood plasma or serum and the mobile 

 phagocytes has now been settled so as to 

 include both factors. The body fluids and 

 the blood serum chiefly carry, as a result 

 of immunization, dissolved substances 

 which act at one time by neutralizing tox- 

 ines that are themselves injurious and at 

 another in sensitizing bacteria and other 

 corpuscular bodies so that they may be en- 



