20 PARASITES IN GENERAL 



when introduced into virgin territory in Uganda is a good ex- 

 ample of the results which may come from such an introduction: 

 in districts where sleeping sickness has existed for a long time 

 the death rate from it is often only two or three per thousand, 

 whereas in one district in Uganda the population was reduced 

 from 300,000 to 100,000 in about seven years. It is not im- 

 probable that the extinction of many of the striking types of 

 animals which dominated the earth in past geologic ages may have 

 been brought about by the sudden appearance of or exposure to 

 new and deadly parasites; only those forms of life which were 

 able to resist the onslaught of the parasites remained to continue 

 the course of evolution. 



This leads us to a consideration of the remarkable facts of 

 immunity. The power of the blood of vertebrate animals to 

 react against invading organisms or poisons by producing sub- 

 stances which will destroy them is one of the most wonderful 

 adaptations in all the realm of nature. Though the details of 

 the reaction are still unknown, the chemical substances concerned 

 still undetermined, and many of the influencing factors not yet 

 understood, yet the progress in our knowledge of the mechanism 

 of acquired immunity has taken great strides since Pasteur 

 first placed the development of immunity on a scientific basis 

 not quite 40 years ago. There are several ways in which the 

 body may react against parasites. One method is by the ac- 

 tivity of the large free-moving white blood corpuscles, which 

 actually capture and devour the parasites after the manner of 

 predaceous protozoans. This, of course, can be done only in case 

 of very small parasites, such as bacteria and Leishman bodies. 

 Apparently the parasites first must be rendered digestible to the 

 white blood corpuscles by the presence of an accessory substance 

 in the blood, known as an opsonin. This substance may be 

 thought of as acting like a sugar coating on a bitter pill, though its 

 effect is more analogous to that of cooking on starch, i.e. increased 

 digestibility. Opsonins are normally present in the blood but 

 increase as a reaction to the presence of parasites^ The degree 

 of development of opsonins in the blood, and consequent power 

 of the white blood cells to capture and digest parasites, is known 

 as the opsonic index. 



Sometimes a number of cells work together to form a capsule 

 around larger parasites, thus walling them in and limiting the 



