HEMAGGLUTINATION 45 



myocarditis virus and fowl plague have been less thoroughly studied 

 from this standpoint. 



These viruses may also be divided into two groups in respect to their 

 behavior toward receptors. In one group the virus-red cell combination 

 may be dissociated only by changing the conditions under which ad- 

 sorption took place (pH, salt concentration, etc.) and there is no evi- 

 dence of any destruction of receptors by virus. All of the agents belong 

 to this group except the MNI viruses, strain 1233 and fowl plague. In 

 none of these cases has it been shown that the receptor site on the red 

 cell is similar or related to the point of virus attachment of the natural 

 host cell. Hence it is not clear whether red cell-virus reactions in any 

 of these cases are specific in the sense of resembling part of a natural 

 process. 



The second group under this scheme (MNI viruses, 1233 and fowl 

 plague) is specifically characterized by the ability of the adsorbed virus 

 particles to spontaneously elute from cells and to exhaust the cell of 

 virus receptors (Hirst, 1942, Burnet, 1945). Though the MNI viruses 

 are heterogeneous in many respects their behavior toward receptors is 

 such that they may be discussed as one group. Two main problems of 

 biological interest are involved: (1) the importance of cell receptors in 

 infection and (2) the role of receptor destruction in infection. 



A wide variety of cells, both red cells of many species as well as the 

 host cells in laboratory infections, readily adsorb viruses of the MNI 

 group. Very likely the adsorbing group in all cases is of similar general 

 character but not identical. Any of the viruses of this group can destroy 

 the cell receptors for the entire group. In addition a number of bacteria 

 manufacture an enzyme or enzyme complex which can destroy these 

 same receptors, either on red cells or on living cells of other tissues 

 (Burnet, McCrea, and Stone, 1946, Stone, 1947). Destroyed receptors 

 are regenerated by hving cells but not by red cells. Influenza viruses, 

 among others, are adsorbed to and multiply in the cells of the chorioal- 

 lantoic membrane of chick embryos. When the receptors of these cells 

 are destroyed by a bacterial enzyme much larger amounts of virus are 

 required to initiate an infection, so much more that it seems likely that 

 influenza virus cannot infect a cell on which the receptors have been 

 destroyed. This is the best evidence so far that receptors are necessary 

 for infection. 



The major effort in this field has been the study of the enzymatic or 

 receptor-destroying role of these viruses. The idea that receptor de- 

 struction is an enzymatic process has gradually come to be accepted. 

 The discovery of receptor-destroying activity in bacterial filtrates, the 



