Bio-synthetic gland

From Memory Alpha, the free Star Trek reference.

Bio-synthetic glands are 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.