A substance (VHI) exists in human plasma which inhibits the agglutination of red cells by influenza virus and is distinct from influenza antibody. When plasma is fractionated by alcohol in the cold the VHI comes out mainly with a mixture of lipid-free alpha and beta globulins (fraction IV-4). On further fractionation the activity comes out with a fraction consisting mainly of beta1 globulin (fraction IV-7). Boiling fraction IV-4 or IV-7 after considerable dilution brings about a large increase in the amount of VHI, much more than can be detected in the original plasma. A similar VHI has been extracted from the ghosts of fowl red cells.

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