IMMUNOLOGY 873 



trypanosomiasis, by the use of stained thick-blood films (see Barber, 

 1936). From the great mass of literature only the tests which have 

 been perfected or show considerable promise will be mentioned. De- 

 tailed protocols and specific methods of procedure can be found in 

 W. H. Taliaferro (1929) or in some of the more recent articles. 



A. Specific hnmunologkal Reactions. — The reactions between antigen 

 (either complete or haptene) and antibody are so specific that, within 

 certain limits, the presence of a suspected antigen can be ascertained 

 with a known antibody, or, vice versa, a suspected antibody can be 

 verified with a known antigen. Both have been used in diagnosis. When 

 the invading organism liberates some antigen, either whole or partial 

 (haptene) into the blood, sputum, urine, and so forth, the antigen may 

 be detected and identified by its reaction with a high titer immune serum, 

 generally prepared in the laboratory. Sometimes, if the organism iso- 

 lated from an infected host cannot be fully identified by morphological 

 criteria, it may be further classified in this way (see section on im- 

 munological methods of classification). Or, if the invading organism 

 during infection stimulates the formation of a specific antibody in the 

 blood, it may be identified by its reaction with a known antigen which 

 is prepared from the organism in the laboratory. In the Protozoa only 

 the last type of reaction has been extensively used. 



The specific complement fixation test is one of the most highly stand- 

 ardized laboratory tests. It is based on the fact that antibody will combine 

 with antigen, and the resultant sensitized antigen will then combine fur- 

 ther with complement (a heat labile component of serum), but neither 

 antigen nor antibody will combine with complement alone. In practice, 

 serum suspected of containing an antibody is first heated at 56° C. for 

 twenty minutes to inactivate the complement which it also contains and 

 then is added in varying proportions to a known antigen. To such mix- 

 tures, known quantities of complement (generally fresh guinea-pig 

 serum) are subsequently added. The actual fixation of complement gives 

 no visible sign, but is tested by adding to the system at this point a 

 suspension of red blood cells which have been previously sensitized with 

 their specific lysin (sheep cells and antisheep lysin are generally used). 

 Obviously, if the complement was previously fixed, there will not be 

 enough left to lyse the sensitized cells. In terms of the original test, 

 if the red blood cells undergo lysis and their hemoglobin colors the 



