MECHANISM OF IMMUNITY 149 



animal actually stimulates this phagocytosis, for Laveran and Mesnil 

 found that if the serum of such an animal was mixed with trypanosomes 

 and injected into the peritoneal cavity of a rat, there appeared in the 

 peritoneal fluid numerous leucocytes which devoured the trypanosomes 

 with avidity. If the trypanosomes were injected alone, this phenomenon 

 was not observed to anything like the same extent. Levaditi and Mutter- 

 milch (1911) showed that the serum affected the trypanosomes in such a 

 way that they attached themselves to the leucocytes. This was inde- 

 pendent of the actual process of phagocytosis, for it was found that 

 attachment to killed leucocytes also occurred. It was shown by Mesnil 

 and Brimont (1909) that if immune serum were allowed to act upon 

 Trypanosoma lewisi a change took place, so that the trypanosomes were no 

 longer able to infect rats even if they were carefully washed free of serum. 

 It would thus appear that the protective action of the serum is a result 

 of its power of causing the trypanosomes to attach themselves to the leuco- 

 cytes which then engulf them. The serum of animals which are immune 

 to Trypanosoma lewisi also has the property of causing trypanosomes 

 to become agglutinated into clumps when blood containing them is 

 mixed with the serum (see p. 452). The presence of agglutinins in the 

 serum has been shown to occur in the case of other trypanosome 

 infections. 



Another property which the serum may acquire is that of producing 

 cytolysis, or the gradual swelling up and dissolution of trypanosomes 

 exposed to its action. It was shown to occur in the case of infections 

 of animals with the pathogenic trypanosomes by Levaditi and Mutter- 

 milch (1909), amongst other observers. They also demonstrated that the 

 serum acquired the property of deviating the complement, a reaction 

 which has found a practical application in the diagnosis of trypanosome 

 infections (see p. 452). It seems evident that recovery from any infection 

 is dependent on the development of antibodies in the blood, which act 

 upon the particular parasites in various ways. This action of the serum 

 of an immune animal is specific for the parasite which stimulated its 

 production. On this account serological tests, which are similar to the 

 inoculation tests referred to above, have been employed as a means of 

 differentiating parasites. If the serum of an immunized animal behaves 

 towards an unidentified trypanosome as it does towards the one which 

 caused the immunity, then, provided that there is morphological similarity, 

 it is concluded that they are identical. On the other hand, it is main- 

 tained by some that, in spite of morphological identity, if the serum fails 

 to act it is proof of a specific distinction. It is possible that a natural 

 recovery would never take place unless antibodies were produced, and 

 that a parasite would continue to multiply continuously till the host 



