“…Hellstrom (1969, 1970) have suggested that during the progressive phase of growth of the MSV induced sarcoma, anti-tumour immunity in vivo is blocked by factors in the serum, perhaps antigen-antibody complexes (Sjogren et al, 1971). The suggestion was made that these blocking factors themselves provided the environment which allowed tumour growth in the face of anti-tumour directed immunity (Halliday, 1972;Hellstrom, 1969,1970;Baldwin, Price and Robins, 1972). In an analogous fashion, it has been suggested that a cell which blocks T lymphocyte activity in a nonspecific manner may be in some way be responsible for tumour progression (Gorczynski, 1974c).…”