SEVERAL reports have concerned themselves with specific suppression (blocking) of tumour-immune responses by serum factors from tumour-bearing animals1-3, and nonspecific depression of T lymphocyte responses in tumour-bearing mice has also been described4,5. Evidence exists that specific blocking of lymphocyte-mediated immunity occurs by means of either free antigen6 or antigen-antibody complexes7. It has now been suggested that nonspecific suppression of lymphocyte responses could also occur by means of immune complexes8, if one assumes that both T and B lymphocytes have Fc receptors for the antibody region of such complexes. There is a growing body of evidence for Fc receptors of T cells9,10. We have investigated the possibility that the nonspecific depression of the T lymphocyte phytohaemagglutinin (PHA) response seen in BALB/c mice carrying tumours, induced by Moloney sarcoma virus (MSV) in the progressive phase of growth4, is caused by similar factors to those which cause specific blocking in this system. Our results suggest that while tumour-specific blocking can be mediated by free antigen, as well as antigen-antibody complexes, nonspecific blocking of T-cell responses is mediated only by the latter.
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