How did scientists turn one immune cell into a drug factory?

Scientists cheated the immune system's own factory by making cells that never stop producing one chosen antibody. A monoclonal antibody is an antibody made by a cell line cloned from a single white blood cell, so all of it traces back to one parent cell. Because they are identical, they bind one particular target, called an epitope. Polyclonal antibodies, by contrast, are mixtures from many cell lines, each binding its own epitope.

The dream is old. In the early 1900s Paul Ehrlich imagined a "magic bullet", a compound that would hit a disease-causing organism and nothing else, and could deliver a toxin to it. For decades antibodies were studied through abnormal single antibodies made by the cancer multiple myeloma, but nobody could produce identical antibodies against a target of choice.

In 1975 Georges Köhler and César Milstein fused myeloma cells, which grow without limit, with B cells, which make a specific antibody. The fused cells, called hybridomas, were immortal and produced antibodies to known antigens. They shared the 1984 Nobel Prize with Niels Kaj Jerne.

The first drugs had a flaw: mouse antibodies injected into people triggered an immune response and were quickly removed from the blood. In 1988 Gregory Winter and his team pioneered ways to humanize antibodies, cutting the reactions that many of them caused. Today such drugs work by blocking a target molecule, by inducing apoptosis in cells carrying it, or by modulating signalling, and they are costlier to manufacture than small molecules.