A scan of Susanna Leijten's upper chest showing the thymus
The thymus was a gland located in the upper chest of a Human body. It played an important role in the Human immune system.
The parasitic Tarchanne cells were located in the thymus after infecting a host, using the immune system to spread genetic instructions that would turn the host into a full member of the species.
Doctor Beverly Crusher successfully removed the parasite from Susanna Leijten's and Geordi La Forge's thymuses after they were infected by the lifeform. (TNG: "Identity Crisis")