24 S. G. ANDERSON 



The size of the hemagglutinating particle has been determined by centri- 

 fugation and filtration studies, giving a particle diameter in the case of 

 influenza of 80 to 100 nuz (Lauffer and Miller, 1944; Friedewald and Pickels, 

 1944; Elford et al, 1936; Sharp et al, 1944; Knight, 1946). 



2. Influence of Environmental Conditions 



a. Temperature. This is not critical, active agglutination being evident 

 from 4 to 37°C. According to Miller and Stanley (1944), the hemagglutinin 

 titer increases with increasing temperature, but, in general, the most im- 

 portant effect of higher temperature is to accelerate the process of elution. 

 This may result, particularly with Newcastle disease virus, in the appearance 

 of a prozone, if pattern-testing is being used. Sedimentation of the cells is 

 more rapid at higher temperatures and this may be desirable with slowly 

 sedimenting mammalian cells. 



b. Ionic Environment. In 5 % glucose solution, PR8 does not adsorb to 

 red cells. Lowell and Buckingham (1948) studied this system, adding various 

 concentrations of sodium chloride to 5 % glucose solution. Adsorption of 

 PR8 virus to red cells did not occur at concentrations below 0.003 M and 

 hemagglutination failed below 0.043 M. The highest concentration tested, 

 0.583 M, allowed full agglutination. Strain LEE behaved similarly (Daven- 

 port and Horsfall, 1948) and Newcastle disease virus was shown to require 

 an adequate ionic concentration by Sagik and Levine (1957). Burnet and 

 Edney (1952) found that, for salts with monovalent cations, concentrations 

 around 0.01 M were needed to allow hemagglutination by active virus. 

 Divalent cations were more active, the end point being 0.004 M to 0.007 M. 

 The nature of the anion used was immaterial, apart from a minor effect seen 

 with calcium deionizing agents (Edney, 1949). 



In influenza virus the optimal range of pH for hemagglutination is 6 to 8 

 (Miller and Stanley, 1944). With Newcastle disease virus, Sagik and Levine 

 (1957) found optimal hemagglutination at pH 5.8 to 6.2, but good agglutina- 

 tion occurred over the whole range from pH 5 to 9, and also from 2 to 3. 

 There were adsorption minima at pH 4 and 10.5. 



3. Influence of Species of Red Cell 



The red cells most widely susceptible to agglutination by myxoviruses are 

 those from chicken, man, and guinea pig. Many other species provide cells 

 agglutinable by some strains of virus but the type of agglutination varies. 

 Comparative studies have been made by Clark and Nagler (1943) and Chu 

 (1948c). 



Newcastle disease virus has an unusual range of susceptible cells, Winslow 

 et al. (1950) finding that the most susceptible species were cow, horse, and 

 sheep. There were differences between different strains of NDV. 



