636 Immunity and Resistance 



Introduction of an antigen into an animal induces the appearance of 

 antibodies which react specifically with that particular antigen (homol- 

 ogous antigen). Introduction of an antigenic complex (e.g., Protozoa, 

 bacteria) induces the appearance of various antibodies corresponding to 

 the different antigens of the complex. Some of these antibodies will react 

 only with the particular microorganism involved. Others, induced by 

 group antigens, will react also with related microorganisms which con- 

 tain such antigens. Antigen-antibody reactions of the latter type are often 

 termed group reactions. Such group reactions may form the basis of a 

 cross-immunity, in which an animal immunized to one strain of parasites 

 shows a detectable immunity to a related strain. Antibodies, which are 

 associated with the globulin fraction of the serum proteins, are evidently 

 proteins. On the basis of their reactions with antigens, they are usually 

 termed agglutinins, precipitins, lysins, and opsonins, although the uni- 

 tarian theory holds that a single antibody produces the various reactions 

 under appropriate conditions. True antitoxins, comparable to those in- 

 duced by bacterial exotoxins, have not been demonstrated in animals 

 infected with Protozoa. The complement-fixation reaction, involving 

 "complement-fixing" antibodies, is discussed below. 



Under suitable conditions, a particular antibody and its homologous 

 antigen will react in a characteristic fashion. A precipitin reaction in- 

 volves the "precipitation" of a non-cellular antigen by a specific precipitin 

 in the presence of an electrolyte. An agglutinin reaction involves agglu- 

 tination of a cellular antigen (bacteria. Protozoa, etc.) by a specific 

 agglutinin under similar conditions. In agglutination of Trypanosoma 

 equiperdum, for example, the flagellates form clumps visible macroscop- 

 ically (138). Under the microscope, the flagellates appear in characteristic 

 rosettes, since the bodies stick together more readily than the motile 

 flagella. Lysis, which also involves a cellular antigen, may bring about 

 disintegration of Protozoa. In lysis of Bodo caiidatus, motility is first re- 

 duced and then the flagellates round up, become transparent and finally 

 disintegrate (140). A lysin, unlike precipitins and agglutinins, acts on the 

 homologous antigen only in combination with complement. A heat-labile 

 complex of substances, complement (or alexin) is found in normal serum 

 as well as in serum from immunized animals. An antiserum containing 

 a lysin loses its lytic activity if it is heated (e.g., for 15 minutes at 56° C). 

 Reactivation is produced by adding a suitable amount of normal serum 

 and thus restoring complement. The opsonic effect, also dependent upon 

 complement, is expressed as an increased phagocytic activity against the 

 homologous antigen. More than one of these various antibodies are to be 

 expected in animals infected with a given parasite. Rabbits infected with 

 Trypanosoma cruzi develop precipitins, agglutinins, lysins, and comple- 

 ment-fixing antibodies (154). Likewise, monkeys infected with Plasmo- 

 dium knowlesi (69) develop agglutinins, complement-fixing antibodies 



