SEROLOGICAL METHODS IN THE STUDY OF PROTOZOA 413 



serums (hence, antiserums) and are postulated in terms of what 

 they do rather than what they are (precipitins, agglutinins, etc.). 

 Some immunologists believe that there are only two fundamental 

 types of antibody response: (i) to antigenic poisons (antitoxins) 

 and (2) to foreign proteins (precipitins, agglutinins, complement 

 fixing, etc. ) ; in the latter case, the particular result elicited depends 

 upon the particular set of conditions under which the serum is 

 tested. 



The chief characteristics of the specific serological reactions 

 dealt with in this section may be briefly summarized as follows : 



The agglutination reaction takes place when an antiserum is 

 mixed with a specific test antigen which is always a suspension 

 of discrete particles such as whole protozoa, bacteria, blood cells, 

 etc. The reaction is evidenced by the clumping of the test antigen. 

 As the clumps increase in size they generally precipitate. 



The precipitation reaction takes place when an antiserum is 

 mixed with (or layered under) a specific test antigen which to be 

 satisfactory should be a clear solution. It is evidenced by the 

 formation of a general precipitate when the two reagents are mixed 

 or of a "ring" of precipitate when one is layered on the other. 



The complement fixation reaction takes place when a solution 

 of an antigen is mixed with its specific antiserum in the presence 

 of complement (a thermolabile non-specific component of normal 

 serums, guinea-pig serum, is generally used) but is not evidenced 

 by any apparent change within the test tube. Consequently, after 

 a sufficient incubation period, another antigen-antibody complex, 

 i.e., sensitized red blood cells, is added as an indicator. If the 

 complement fixation reaction has taken place, the non-specific 

 complement is fixed and cannot be utilized by the sensitized red 

 cells, hence there will be no change in the test tube. On the 

 contrary, if the reaction has not taken place, the non-specific 

 complement is free, combines with the sensitized red cells and 

 lysis takes place, thus liberating the hemoglobin of the red cells 

 (see below). 



Lysis causes the destruction of an organism by disintegration 

 and is brought about by the action of at least two components of 

 the antiserum: a thermostabile, specific component (amboceptor, 

 sensitizer) and a thermolabile, non-specific component (comple- 

 ment, alexin). Furthermore, the parasite can be sensitized by 

 in vitro contact with the amboceptor so that lysis occurs when 



