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.