646 Immunity and Resistance 



demonstrated with sera from rats infected with T. leiuisi (103). Practical 

 application has been fairly successful in the diagnosis of dourine in 

 horses. Although the diagnostic value of the reaction has not been de- 

 termined, erythrocytic stages of avian malarial parasites (110) and P. 

 knowlesi of monkeys (79) are agglutinated by homologus antisera. 



Precipitin tests 



The test antigen is usually prepared as an extract of the suspected 

 parasite. Various dilutions of the antigen, in physiological salt solution, 

 are then tested with serum from the host. Group reactions may be 

 eliminated by increasing the dilution of the antigen. Consequently, a 

 reaction with the antigen in high dilution has the same general signifi- 

 cance as agglutination with high dilution of the test serum. In addition 

 to demonstrating specific antibodies in the blood of the host, the test 

 also may detect antigens of the parasite in body fluids of the host, as 

 in Trypanosoma equiperdum infections of laboratory animals (131). In 

 this case, a known antiserum is tested with material from the host, serving 

 as the test antigen. 



The precipitin reaction has been applied to diagnosis of dourine in 

 horses, and has been tried also in diagnosis of human trypanosomiasis 

 (119, 158, 159). Muniz (119) has found the test reliable for active cases 

 of Chagas' disease, although much less sensitive than the complement- 

 fixation reaction in chronic cases. Precipitin tests have proved positive 

 for well developed Entamoeba histolytica infections in cats, although 

 negative for early infections and dying animals (194). Good results have 

 been reported also for malaria (142, 169). Group reactions, common to 

 sera from patients with P. falciparum and P. vivax, have been noted. 

 However, more intense reactions are obtained with the homologous 

 antigen (142). 



Complement-fixation tests 



Specific complement-fixation depends upon the fact that an anti- 

 gen and its homologous complement-fixing antibody will "fix," or com- 

 bine with complement. If either the antigen or the homologous antibody 

 is absent, complement is not fixed. The results are read in terms of an 

 indicator, the so-called hemolytic system. In carrying out such a test, 

 measured quantities of the test antigen, the test serum (heated to in- 

 activate the complement), and complement (in normal guinea pig serum) 

 are added, in physiological salt solution, to a serological test tube. After 

 incubation, a suspension of red corpuscles and an appropriate amount 

 of inactivated serum containing homologous hemolysin are added to the 

 tube. The red corpuscles and the hemolytic serum constitute the "hemo- 

 lytic system." The complete inixture is incubated and later examined 

 for effects on the red corpuscles. A settling out of the corpuscles without 



