34 S. G. ANDERSON 



The most suitable method of preparing indicator virus varies from strain 

 to strain. Francis's original method of heating allantoic fluid at 56°C. for 30 

 minutes is satisfactory for LEE and many other strains. Others, such as 

 MEL, are not changed to the indicator state by this treatment. Stone (1949b) 

 developed the method of heating allantoic fluid adjusted to pH 8.5 with 

 citrate or some other calcium deionizing agent added. In the same paper, 

 Stone describes a reversible indicator state which she induced in one strain of 

 PR8 and WSE. A variety of other methods may produce similar changes with 

 certain strains; these include treatment with periodate ion and trypsin 

 (Hoyle, 1952; Stone, 1949a). 



Indicator viruses agglutinate red cells but do not elute; in general, they 

 occupy a position in the receptor gradient a little below the position occupied 

 by the active virus; they can be removed from red cells by the action of 

 RDE or of an active virus lower in the gradient. They have no enzymatic 

 action on soluble mucoid inhibitors; Hirst (1948b) and Burnet (1948b) have 

 suggested that indicator virus is inhibited by mucoid because it cannot 

 destroy the mucoid receptors. Anderson et al. (1948) and Stone (1949b) pointed 

 out that this simple explanation may not be complete or correct. For example, 

 at 0°C, where enzyme action is negligible, active virus should be inhibited by 

 mucoids, but this is not the case. Smith and Westwood (1950) have also 

 argued that loss of enzyme does not, ipso facto, make a virus an indicator 

 virus. 



Active virus reacts with red cells, changing the receptors and completely 

 eluting. Indicator viruses, at least of LEE and MEL, do not elute, but have 

 a second reaction with fowl red cells leading to a firm union of a proportion 

 of the virus particles with the red cells, a union which cannot be broken by 

 RDE. This reaction requires 15 to 30 minutes at 37°C, or a longer time at 

 lower temperatures. It does not occur with human cells (Burnet, 1952b). 



F. Inhibitors of Hemagglutination 



1. Specific Antibody 



Specific antihemagglutinating antibody appears following infection or 

 vaccination with most influenza strains. Its specificity corresponds approxi- 

 mately to that of active immunity, but there is still much uncertainty about 

 the details of cross-reactions among strains and antisera. The most recent 

 discussions are by Jensen and collaborators (1957; Jensen and Petersen, 

 1957; Jensen, 1957). The antihemagglutinin response of a human depends 

 largely on the serological character of his first experience of influenza 

 (Davenport and Hennessy, 1956). 



Specific antibody is stable at 62°C. for 30 minutes. It is not destroyed by 

 incubation with RDE, influenza virus, or moderate concentrations of 

 periodate ion. 



