3. 4. 5.

In a previous communication (Laskov et al., 1968a), the level of a human natural cytotoxic antibody active against Landschutz ascites tumor cells was determined in normal sera. It was found that the cytotoxin titer was higher in sera taken from individuals with blood groups A and 0 as compared to sera from groups B and AB. This suggested that the antigenic determinant on the tumor cell may be chemically related to the B blood group substance. In the present report the chemical nature of this antigenic determinant was studied by inhibiting the cytotoxic and agglutinating activity of human serum with various simple sugars. These studies indicate that D-galactose is an essential part of the antigen involved. Absorption studies showed that a B-like antigenic determinant is indeed present on the ascites tumor cell, and that the natural anti-B-isohemagglutinin does not participate to a significant extent in the cytotoxic r:a:ttnti.In addition, the haptenic properties of some of the sugars were demonstrated by their capacity to elute the cytotoxic antibody from antibody-coated tumor cells.

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1. The cytotoxic antibody in normal human serum, active against mouse tumor cells, was identified as a 19S macroglobulin by means of Sephadex gel filtration, DEAE‐cellulose chromatography and density‐gradient centrifugation procedures. 2. A purified IgM fraction was isolated from normal human serum. The cytotoxic activity of this fraction, as tested against mouse tumor cells, was enriched 20–80 times over that of the unfractionated serum. The purified IgM showed mainly one precipitin line on immunoelectrophoretic analysis. In the ultracentrifuge the 19S peak comprised about 85% of the total protein.

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