Immune responses against the C3H mouse ascites tumour BP8 were studied in vivo by measuring the proportion of an inoculum that was killed in allogeneic C57BL animals. Killing was assayed by labelling the cells with laaIIJdR; the la61 liberated from dead cells was excreted, and the proportion of label (and thus the proportion of cells) remaining was measured by whole-body counting. Cell loss from the primary immune response began about a week afer intraperitoneal injection of lo' cells, and was accelerated by addition of 107-100 heavily irradiated cells to the inoculum. By contrast, it began immediately in immunized mice. Labelled cells irradiated with 10,000 R of X-rays did not begin to lose laaI until 2 days after irradiation, and so could be used to investigate the immune status of ati animal without fear of killing it.
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