Immunity and Resistance 645 



sponse to a mild infection or in the early chemotherapeutic suppression 

 of primary attacks. Conversely, artificial stimulation of the defensive 

 mechanism in conjunction with administration of malaricidal drugs may 

 intensify effects on the parasites. As reported by Garcia (77a), injections 

 of tetanus toxoid following atebrin or chloroquine therapy have reduced 

 the relapse rate with P. vivax from an average of 90 to 9.5 per cent and 

 that with P. falciparum from 90 per cent to zero. 



SEROLOGICAL DIAGNOSIS OF 

 INFECTION 



Diagnosis of a protozoan infection is usually based upon detection 

 of the parasites in body fluids or other materials. This method works well 

 enough when the parasites are present in reasonably large numbers. In 

 early infections and in chronic and latent stages, however, the usual 

 laboratory examination becomes time consuming and often fruitless. 

 Such difficulties have stimulated attempts to apply antigen-antibody 

 reactions to diagnosis of protozoan infections. These tests are based upon 

 the principle that the infected animal will sooner or later develop anti- 

 bodies which react specifically with the causative organism. 



In the usual procedure, a suitable antigen — a suspension of the para- 

 sites for an agglutinin test, or an extract for a precipitin or a complement- 

 fixation test — is prepared from the suspected organisms. Serum from the 

 infected animal is then tested with this antigen. A positive reaction, after 

 elimination of possible group reactions, indicates that the host has 

 developed antibodies against the particular parasite. A positive test, with 

 corroboratory clinical evidence, may thus be considered presumptive 

 evidence of infection. 



Agglutinin tests 



A suspension of the parasite in physiological salt solution is mixed 

 with dilute serum from the suspected host. Agglutination becomes in- 

 creasingly significant as the dilution of the serum is increased. Agglutina- 

 tion at a low dilution may represent merely a group reaction. Since 

 group agglutinins are usually present in concentrations lower than those 

 of antibodies specific for the homologous parasite, they are gradually 

 eliminated with increasing dilution of the serum. A positive test at high 

 titre thus indicates that the host has produced specific agglutinins for 

 the test antigen. 



The results obtained with kala azar have been contradictory. Some 

 tests have been fairly successful, while others have not been clear cut. 

 Caronia (41) demonstrated agglutinins in children infected with L. dono- 

 vani, but concluded that the titre was too low for diagnostic purposes. 

 More recently, however. Row (141) has obtained well marked agglutina- 

 tion of flagellates from cultures. Agglutination of trypanosomes was first 



