Leukocytes and macrophages, obtained from fibroma-immune rabbits and added to immune serum-fibroma virus mixtures, significantly increased the neutralization of fibroma virus as compared with immune serum alone. Immune cell suspensions from peritoneal exudates, regional lymph nodes, buffy coats, spleen, and liver were all effective in inhibiting fibroma virus. Approximately 2000 to 4000 immune cells/mm.3 were necessary to cause an effect but no particular cell type could be implicated as responsible for the inhibition of fibroma virus. Normal cells did not consistently and significantly inhibit fibroma virus and cells from rabbits immunized with other viruses did not inhibit fibroma virus.

Studies of the mechanism of action of the immune cells revealed: (a) that living cells were essential; (b) that normal cells, sensitized with immune serum, did not simulate the effects of immune cells; (c) that immune cells contained less preformed neutralizing antibody than an equivalent volume of immune serum, and (d) that inhibition of fibroma lesions was not the result of viral interference.

It is suggested that the fibroma-neutralizing effect of immune cells is related to intracellularly placed antibody or to cellular transfer of an ability to form specific antibody in recipient animals.

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