The Song of the Cell: An Exploration of Medicine and the New Human

by Siddhartha Mukherjee

killer T cells: T lymphocytes that recognize virus-infected cells and souse them with toxins until they shrivel and die, thereby purging the microbe taking refuge there. These cytotoxic (cell-killing) T cells, brandished a particular marker on their surfaces: CD8, a type of protein. The peculiar thing about these CD8-positive T cells, Zinkernagel and Doherty discovered, was that they had a capacity to recognize viral infections only in the context of the self. Consider that thought: your T cells can recognize virally infected cells only if they come from your body, not someone else’s.III A second feature of killer Ts was just as puzzling. Although a CD8 T cell could recognize a cell from the same body, it killed only infected cells from the same body. No viral infection, no kill. It was as if the T cell was capable of asking two independent questions. First: Does the cell that I am surveying belong to my body? In other words, is it self? And second: Is it infected with a virus or a bacterium? Has the self been changed? Only if both were true—the self and infection—would a T cell kill its target. In short, T cells had evolved to recognize self, but an altered self that happens to harbor an infection. But how? Using genetic techniques, Zinkernagel and Doherty tracked the detection of the self to a set of molecules called MHC class I.IV