Burnet's concept of immunosurveillance1 is one of the most important hypotheses of our era. He theorizes that the evolutionary purpose of cell-mediated immunity is to annihilate mutant neoplastic clones. Basic to Burnet's concept is the statistical frequency with which a neoplastic cell is produced by an error in nucleic acid replication. If at the time of mutation the cell is well differentiated and incapable of extensive self replication, no new cell line commences. On the other hand, if the error occurs in a stem cell, one of three events may occur:
The genetic change is so inconsequential that structurally and functionally the cell cannot be distinguished from normal.
The mutation is lethal and the cell dies, thereby terminating the neoplastic line.
There is an intermediate degree of genetic change which establishes a neoplastic clone. These cells are somehow different, structurally or functionally, or both, from the corresponding line of