Published evidence suggests that circulating antibodies play a simpler and more direct role in leukotic homografts than appears to be the case with other types of tissue.

Antibodies were produced against the C57BL leukosis E.L.4 following intensive immunization of strains A, BALB/c, and C3H. An empirical measure of the efficacy of the response may be obtained from the hemagglutinin titer. With the systems described, sera giving a hemagglutinin titer of over 4,000 for A cells or over 1,000 for cells of other strains were found satisfactory. These high-titered sera injected into mice of the three resistant strains gave passive immunity to E.L.4 over a wide range of cell doses.

Injection of such sera into C57BL mice resulted in the rapid removal of all hemagglutinating activity, but a considerable degree of protection was obtainable in mice of this strain. The antibody concerned in this case was termed anti-X.

Sera produced against antigens other than E.L.4 contained hemagglutinins but no demonstrable anti-X. Such antibodies would protect mice of heterologous strains but less efficiently than sera in which anti-X was present and failed to protect C57BL mice.

The nature of the X component is discussed. It is doubtful if it is to be regarded as the product of a mutation at an H locus. Further speculation must await data on the frequency of such components in leukoses, particularly in those of recent origin.

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