Each chick and human red cell contains approximately 300 sites capable of attaching influenza virus particles. These correspond to an area representing approximately 2 per cent of the red cell surface.

Although the rate of attachment of PR8 to red cells is not diffusion-limited, when calculated on the basis of the total cell area, it does approach the theoretical maximum for interaction between the virus and the fraction of the cell area known to contain attachment sites.

It is demonstrated that the virus attachment can initiate a spreading disturbance on the red cell membrane which extends over an area far exceeding that covered by the attached virus and which leads to the destruction of receptor sites. This process does not involve cyclic virus attachment, elution from the cell, and reattachment to another site.

Practically all the receptor sites on a cell are destroyed before any virus is liberated into the medium.

The spontaneous elution of virus from red cells within 24 hours at 37° C. requires a threshold value of at least 3 and less than 5 virus particles per cell.

Parallelisms between the spontaneous elution reaction and the phenomenon of lysis-from-without in the bacteriophage system are demonstrated.

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