36 REPORT ON THE DISEASES OF SILKWORMS IN INDIA 



While acquired immunity is of the greatest importance in the 

 study of disease in the higher animals, it does not come into ques- 

 tion at all in connection with silkworms. So far as we know, once 

 a caterpillar is infected with pebrine, it continues infected until 

 it dies — it does not recover and remain immune to further attacks. 

 We have therefore only to deal with natural immunity. 



In order to understand immunity one must know what injury 

 the animal concerned has to protect itself against and how the protec- 

 tion is achieved. Parasites cause hurt to their hosts in several 

 different ways which may be grouped in four main classes : — 

 (1) Mechanical or the destruction of cells and tissues invaded by the 

 parasite ; (2) the " digestion " of the tissues of the host by the para- 

 site ; (3) the lessening of the normal food supply of the host by the 

 parasite absorbing its food either from the digested food in the gut 

 or from the food absorbed by the gut and distributed to the various 

 tissues ; (4) the production of waste products — toxins^-which are 

 poisonous to the host. The ways in which an animal, on the other 

 hand, resists parasitic attacks are more subtle and difficult to deter- 

 mine but there seem to be certain very definite defences in the animal 

 body. In addition to such obvious protection • as, for example, 

 the impervious skin and the action of the digestive juices afford, 

 there are the active and very efficient wandering cells or phagocytes 

 — minute freely moving cells capable of being hurried by the blood 

 stream to any point attacked by parasites and there actually assail- 

 ing the invaders, and if these are small enough taking them inside 

 their bodies and digesting them or causing sloughs of infected tissue 

 to be thrown off. Still more elusive but extremely potent defences 

 are afforded by the germicidal power of blood and the ability of 

 animals to produce substances — anti-toxins— which in some way 

 neutralize the poisonous waste products of certain parasites — the 

 toxins. Other means of defence there are, but enough has been 

 given to show that animals are by no means powerless against 

 -attack. 



We must here, however, stop to note an important feature of 

 parasitism, remembering that we are dealing at present not with 

 bacteria but with protozoa, namely, that the huge majority of para- 

 sites do relatively little injury to the animal in which they live and 

 consequently the whole army of animal defences is not arrayed for 

 their destruction. The efficient parasite is one which causes as 

 little injury to its host as possible — it is one which in the long course 

 of time has arrived at a modus vivendi with its host. When a 

 p arasite produces toxins that are fatal to the host or rapidly des- 

 troys large areas of tissues with fatal results, it is a clear sign that 

 either the parasite and host are comparatively new to one another, 



