IMMUNITY REACTIONS 21 



sphere of their activity; this occurs in the case of Trichinella, 

 filarial worms, larval tapeworms, fly maggots, etc. It occa- 

 sionally happens that enzymes are developed which enable 

 the surrounding cells slowly to digest the parasites thus im- 

 prisoned. 



More important than these physical methods by which the 

 body is able to combat parasites are the biological or chemical 

 methods. One of the most wonderful adaptations in the animal 

 kingdom is the ability of tissues, chiefly the blood and lymph of 

 vertebrate animals, to react against invading cells, whether they 

 be bacteria, Protozoa, blood corpuscles of unrelated animals, or 

 other foreign cells, by producing substances known as anti- 

 bodies which dissolve these cells, or cause them to agglutinate, 

 i.e., clump together and lose any motile power they may have. 

 The living body is also able to destroy the poisonous action of the 

 excretions or toxins of parasites by the development of protective 

 substances known as anti-toxins which form some sort of a chemi- 

 cal union with the toxins. Other poisonous products are ren- 

 dered harmless by the formation of " precipitins " which have 

 the special property of uniting with the toxic substances to pro- 

 duce insoluble and consequently harmless precipitates. Pre- 

 cipitins are found to react against any foreign protein, of para- 

 sitic origin or otherwise, introduced into the body. There is 

 some question as to whether immunity reactions against animal 

 parasites are exactly comparable with those against bacteria, 

 but the difference is probably a matter of degree and not of 

 fundamental nature. The higher organization of Protozoa and 

 of other animal parasites enables some of them to react against 

 the destructive influence of the serum by encysting, or by form- 

 ing spores, and thus they are able to continue their existence in 

 spite of the development of immunity, though in very limited 

 numbers and with limited activity. The result is immunity 

 without sterilization; in other words, although the body becomes 

 more or less completely immune as far as suffering from the 

 effects of the parasite is concerned, yet the parasites, limited in 

 number and activity, still exist within it, and such a host becomes 

 an " immune carrier." With few exceptions protozoan diseases 

 are contrasted with bacterial diseases in this respect.' The 

 gradual development of anti-toxins, precipitins, etc., probably 

 accounts to a large extent for the relative immunity which is 



