To survive long enough to form a tumor, cancer cells must ward off attacks by the body's immune system. Some cancers protect themselves by vacuuming up the amino acid tryptophan, which nearby immune system cells need in order to attack.
Now scientists have discovered that these cancer cells simultaneously pump a poison into their surroundings, killing those immune system cells when they get too close.
These two actions—soaking up tryptophan and dumping out the toxin—are intimately connected, the researchers found. Whenever a pore in the cancer-cell membrane lets in a tryptophan molecule, it ejects some other molecule, like two people passing through a revolving door in opposite directions. Usually, the expelled molecule is the poison.