Over the past decade, biotech companies have introduced a string of protein-based antibody drugs, which mimic the action of the body's own immune system to combat conditions like cancer and arthritis. Antibody drugs can zero in precisely on misbehaving cells and often enlist the aid of our own antibodies to force a disease into retreat. But there's a price for their effectiveness. Antibody drugs are based on large molecules, so they have a hard time getting into the bloodstream and into cells.

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