648 Immunity and Resistance 



which fix complement in the presence of antiserum (70). Although com- 

 plement-fixation with P. knowlesi or P. gallinaceum antigen in human 

 malaria is a group reaction and false reactions are sometimes obtained 

 with syphilitic sera (7, 83), its practical value as a supplementary method 

 in diagnosing mild infections with P. vivax seems to have been demon- 

 strated (68). However, the test seems to have no value in latent vivax 

 malaria. An interesting outgrowth of these investigations is the demon- 

 stration that P. gallinaceum antigen is effective in complement-fixation 

 tests for Haemoproteus columbae in pigeons (197). Perhaps the relation- 

 ship between Plasmodium and Haemoproteus is closer than is generally 

 believed. 



Tests with human sera from known and suspected cases of toxoplasmo- 

 sis have indicated that complement-fixation may be useful in diagnosis 

 of active toxoplasmosis (196). Complement-fixation tests may be positive 

 also in animals infected with Coccidia (8, 42) but the diagnostic value 

 is uncertain. 



In addition to specific complement-fixation, in which the reaction is 

 dependent upon the presence of a particular antigen and its homologous 

 antibodies, non-specific complement-fixation tests have been used exten- 

 sively in serological diagnosis. In these tests, the test "antigen" bears no 

 apparent relation to the parasite causing the infection under consider- 

 ation. The best known example is the Wassermann reaction, in which 

 the test "antigen" is extracted from normal ox heart. A comparable 

 non-specific test has been tried in diagnosis of kala azar (155, 156), the 

 "antigen" being prepared from a human strain of Mycobacterium tuber- 

 culosis. Although occasional false positives have been obtained in pulmo- 

 nary tuberculosis, this test for kala azar seems to be fairly reliable. 



Skin tests 



Diagnostic skin tests depend upon a cutaneous inflammatory re- 

 action induced by an antigen, introduced either by intradermal injection 

 or by the scratch method, into an animal containing homologous anti- 

 bodies. A positive reaction in man usually involves both immediate and 

 delayed reactions. In lower animals which react at all, an inflammatory 

 reaction usually develops the day after inoculation. The minute reddened 

 area of a negative reaction is readily distinguished from a positive test. 

 Skin reactions to Leishmania have been obtained in rabbits immunized 

 to L. donovani and L. tropica (193) and also in human cases of dermal 

 leishmaniasis (117, 153). Positive tests have been reported also in rabbits 

 immunized to Trypanosoma cruzi (154), in human amoebiasis (147), and 

 in guinea pigs for several weeks after recovery from coccidiosis (88). 

 Preliminary experience with an intradermal test for human malaria has 

 been promising. With antigen prepared from Plasmodium gallinaceum, 

 the test compares favorably with examination of blood films (108). 



