Antibodies bind with great affinity and specificity to their target antigens, allowing them to be exploited in research, medicine, agriculture, and industry (1 –3 ). It is estimated that more than 1,000 antibody-based pharmaceuticals are currently in development, and about 200 of these are already undergoing clinical evaluation in humans. Such widespread use of antibodies would benefit from a safe, convenient, and cost-effective system for large-scale production.