Ehrlich hedged. The cancer cell, he explained, was a fundamentally different target from a bacterial cell. Specific affinity relied, paradoxically, not on “affinity,” but on its opposite—on difference. Ehrlich’s chemicals had successfully targeted bacteria because bacterial enzymes were so radically dissimilar to human enzymes. With cancer, it was the similarity of the cancer cell to the normal human cell that made it nearly impossible to target.

Author: Siddhartha Mukherjee