Effect of Human Blood Serum on Tissue Cultures. III. A Natural Cytotoxic System in Human Blood Serum2

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Abstract
A cytotoxic system in human blood serum has been recognized and shown to consist of a natural antibody, a complement, and divalent cations. The complex formed by this antibody and appropriate cells fixes human but not guinea-pig complement. The complement fixation occurs in the presence of divalent cations at 37° C but not at lower temperatures. The natural cytotoxic antibody in the absence of complement agglutinates strain L cells, and produces mixed agglutination between strain L cells and rabbit, mouse, rat, cat, dog, or cow erythrocytes. Because the antibody agglutinates cells, it must be at least divalent and the receptor sites for the antibody must be situated on the cell surfaces. We believe that this cytotoxic system is part of a heterophil antigen-antibody system and that the cytotoxic antibody is different from Forssman antibody and from the heterophil antibodies found in infectious mononucleosis and serum sickness.