HEMAGGLUTINATION BY ANIMAL VIRUSES 35 



Union between antibody and virus is reversible during a short period after 

 mixing (Isaacs, 1948; Burnet et al., 1945). Antihemagglutinin titers of 

 specific antibody depend a little on the source of the fowl cells used, but this 

 effect is not so pronounced as in the case of Chu inhibitor (Stuart-Harris, 

 1943; Hirst, 1943b). 



2. Chu Inhibitor 



This term is used to denote nonspecific thermolabile factor(s) in serum, 

 inhibiting viral hemagglutinin and sometimes infectivity. 



Hirst (1942a, 1943a) and Burnet and McCrea (1946) described a factor in 

 normal rabbit and ferret serum and in ground ferret lung which inhibited 

 hemagglutination by fresh virus. Similar nonspecific inhibitors have been 

 found in other tissues of humans, animals, and chick embryos. Beveridge and 

 Lind (1946) found in allantoic fluid and yolk sac suspensions a nonspecific 

 inhibitor of mumps virus. This hemagglutinin is also inhibited by the poly- 

 saccharide of Friedlander bacillus type B (Ginsberg et al., 1948). 



a. Relation to Virus. The union between virus and Chu inhibitor is revers- 

 ible and the titer of inhibitor depends on the affinity between cells and virus. 

 Thus, the titer of inhibitor against any one strain of virus varies with the 

 source of fowl cells, being greatest with those fowl cells having the least 

 affinity for virus (Anderson et al., 1946). Probably for similar reasons Chu 

 inhibitor is more inhibitory against recently isolated strains than against 

 classic egg-adapted viruses (Tamm, 1954b; Chu, 1951). 



Chu inhibitor may also prevent infection in certain circumstances. Burnet 

 and McCrea (1946) found that normal ferret sera would prevent infection of 

 the amniotic cavity of chick embryos by BEL virus and protected mice 

 against inoculation with BEL D virus. Chu (1951) described a thermolabile 

 inhibitor in mouse serum which prevented infection of mice by unadapted 

 but not by adapted strains of influenza. This may be the same as the in- 

 hibitor described by Ginsberg and Horsfall (1949) in sera of humans, guinea 

 pigs, mice, and rabbits, which, however, was labile on storage at 4°C. for 

 several weeks. Active virus does not destroy Chu inhibitor (Chu, 1951). 



b. Physical and Chemical Properties. A definitive characteristic of this in- 

 hibitor is its destruction by heat. The temperature of destruction varies a 

 little with the source of inhibitor but is in the range of 56 to 62°C. (Burnet 

 and McCrea, 1946; McCrea, 1946). 



McCrea (1946) precipitated this type of inhibitor from rabbit serum by 

 33 % saturation with ammonium sulfate. He believed the agent to be a 

 gamma globulin. 



Anderson (1948) was able to destroy this inhibitor by treatment of normal 

 human serum with preparations of crude RDE. Mulder and van der Veen 

 (1948) applied this technique to ferret sera, using crude RDE. Chu and 



