The work of previous investigators gives the impression that it is easy to produce sera which both in vitro and upon injection are leukotoxic. At the same time the specificity of these leukotoxic sera for the particular type of cell used as antigen, and even for leukocytes in general, has been doubtful. The methods used have made certain possible factors of error unavoidable. Even careful washing of an organ or suspension cannot render it wholly blood-free, so that it is not surprising that the sera should be moderately hemolytic and hemagglutinative. Pearce has shown that the injection of very small amounts of blood is sufficient to evoke the production of immune hemolysins. When such sera are injected the lesions, as Pearce states, may be due in part to the production of hemagglutinative thrombi, although this hardly seems to apply to the changes in lymphoid tissue described by Flexner. On the other hand, the lymphotoxic effect of hemolytic sera may be due to the lymphocytes injected with the red cells.

Our own experiments indicate that the lymphotoxic and agglutinative factors are to a considerable degree distinct from the hemolytic and hemagglutinative ones, since they can be separated from one another by absorption.

Further evidence is presented that the small thymus cells are biologically related to, if not identical with the lymphocytes derived from lymph glands.

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