Involution of the thymus in tumor-bearing mice and its restoration by PSK.

In Balb/C mice, the size and weight of the thymus and the number of thymus cells were reduced 1 week after the subcutaneous inoculation of Meth A fibrosarcoma. These changes were prevented by the intraperitoneal administration of PSK. In normal mice, the majority of thymus cells are large and the minority are small as demonstrated by analysis with a fluorescence activated cell sorter. In tumor-bearing mice, the number of large cells are decreased and the number of small cells are relatively increased. PSK prevented such a modulation in tumor-bearing mice.