Metastatic spread and "escape" from the immune defenses of the host.

The effectiveness of the host's immune reaction against primary and disseminated tumors depends not only on the magnitude of the host's immune response, such as the number of cytotoxic cells and antibody molecules produced, but also on the capacity of tumor cells to evade destruction. The latter process which has been termed "escape" depends on several factors including intrinsic properties of the tumor cell. In some experimental systems, it was shown that the capacity of disseminated tumor cells to give rise to distant metastases is in part determined by the efficiency of escape. Tumors which in vivo appear to be nonimmunogenic may still carry tumor-specific antigens to which the host responds by making cytotoxic mononuclear cells, but these fail to kill because escape is effective.