VIROLOGICAL METHODS 



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controls to learn if the antibody titer is greater against an egg-propagated 

 antigen than against the egg-products control. If egg-propagated virus 

 has been used as an antigen to produce control antiserum, the problem 

 involved in using this serum as a control against egg products is obvious. 

 If the chicken is the donor of the antiserum, this might be obviated as 

 far as heterologous tissue antigen is concerned, but chicken sera may 

 function poorly in the complement-fixation test for other reasons. When- 

 ever possible, however, it is probably advisable to have reference serum 

 containing antibody against only the virus and/or its products. Some- 

 times this is possible by the use of homologous tissue virus antigens to 

 produce the antibody, e.g., mouse-lung virus to produce antibody in mice. 

 Otherwise, adsorption with heterologous antigen may be employed. 



To avoid excess of either antigen or antibody most workers cross- 

 titrate the serially diluted antigen against a serially diluted antibody as 

 indicated in Table 28. 



For experimental details on this titration, see Chap. IX. 



Both the antigen and antibody used in the above test were strong. 

 Either could have been weak, such as an early convalescent serum or an 

 unconcentrated freshly isolated and poorly adapted viral antigen. In 

 such cases, unless the cross-titration method is employed, negative results 

 might be obtained where they should be positive. 



Peculiarities. Soluble antigen may be difficult to detect without the 

 complement-fixation technic. Artificial active immunization using 

 inactive virus may fail in some instances to call forth the antibody 

 involved in the complement-fixation test, whereas an active infection 

 may more regularly do so. In a number of instances the complement- 

 fixation reaction has a broader spectrum in recognizing antigenic relation- 

 ships among viruses than does the hemagglutination-inhibition test. 



