Bio-synthetic glands were a type of gland present within the body of Borg drones.

In 2374, Seven of Nine's Human immune system began to reject her Borg modifications, including the bio-synthetic glands. In order to save her life, The Doctor had to remove nearly all of Seven's Borg circuitry. (VOY: "The Gift")

Bio-synthetic glands presumably replace an individual's organic glands upon assimilation.
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