414 RESEARCH IN PROTOZOOLOGY 



complement is later added, whereas complement cannot combine 

 either with the test antigen or amboceptor alone. 



Phagocytosis also causes the destruction of an organism and 

 consists in the engulfing of the organism by certain cells of the 

 body whose action is greatly intensified in immunized hosts by 

 opsonins which essentially sensitize the organism. Some immunolo- 

 gists (cf. Wells, 1925, p. 238), who look upon phagocytosis and 

 lysis as being fundamentally the same, consider that a specific 

 antibody sensitizes the invading parasite so that enzymatic lysis 

 occurs, in one case outside the cell, in the other, inside. Levaditi 

 and Mutermilch (1910) found that the in zntro phagocytosis of 

 trypanosomes under the influence of immune serum possessed two 

 phases : ( i ) the attachment of the trypanosome to the leucocyte, 

 and (2) the actual process of incorporation of the parasite by the 

 leucocyte. Although the second phase can take place only with 

 live leucocytes, the first phase can take place between sensitized 

 trypanosomes (parasites treated with immune serum) and leu- 

 cocytes in vitro at 0° C, even if the leucocytes have been previously 

 killed by a three-day sojourn in the ice-box, by heating to 45° 

 55° or 60° C, by successive freezing and thawing or by mechan- 

 ical action. The sensitization of the parasites, i.e., the action of 

 the immune serum on the trypanosomes, falls essentially into the 

 same category as the combination between lysin and specific or- 

 ganism. 



Of these reactions, complement fixation is generally considered 

 the most delicate, but in some cases agglutination is best. On the 

 other hand, the precipitin reaction has the great advantage of 

 simplicity, and can be used with far stronger test antigens than 

 can complement fixation because the anticomplementary factor, to 

 be discussed later, does not enter. In fact, in view of some recent 

 work by C. I. Nelson, in which mutants of the same species of 

 flax have been difYerentiated with the precipitin test, the superiority 

 of the complement fixation test may be principally due to the con- 

 centrated attack that has been made on its standardization. This 

 was a logical outcome of the success attending the non-specific 

 Wassermann reaction in syphilis which utilizes similar technique. 



SEROLOGICAL REACTIONS WITH EXTRACTS OF THE PROTOZOA : 

 COMPLEMENT FIXATION AND THE PRECIPITIN TEST 



A. Summary of methods of preparing test antigens. Inasmuch 



