Abstract
The influenza virus is composed of four major structural proteins: the nucleoprotein and the membrane protein, which are located internally, and the hemagglutinin and the neuraminidase, which are located externally.1 The infected host may respond immunologically by producing antibodies to all four proteins. Since antibodies to the internal proteins appear to offer no protective advantage, our attention is directed largely toward the two external proteins.The hemagglutinin and the neuraminidase are morphologically and antigenically distinct structures on the surface of the virus. Both may independently undergo antigenic mutation. These facts have led to the institution of a completely new system . . .

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