The thymus sits behind the breastbone, in front of the heart, and it is the one endocrine organ that spends its whole functional life shrinking. Largest relative to body size in infancy and still substantial through childhood, it is where immature white blood cells travel to be educated as T cells, screened so that the ones able to attack the body's own tissue are eliminated and only those able to recognize genuine threats are released into circulation. After puberty the thymus begins involuting, its functional tissue gradually replaced by fat, so that by late adulthood only a small remnant remains, still producing a trickle of new T cells but nowhere near a child's output. Its immunological role, long a mystery to earlier physiology, was only firmly established in the middle of the twentieth century.
Hatha yoga's subtle body places it at the body's own midpoint. In Swami Satyananda Saraswati's synthesis in Asana Pranayama Mudra Bandha, the thymic correspondence sits with Anahata, the Heart chakra, the center the tradition reads as love, compassion and connection, and specifically as the hinge between the three lower chakras of survival and appetite and the three upper ones of expression and insight. The immune system's task of distinguishing self from threat and the tradition's reading of the heart as the seat of connection are offered here as two separate accounts of the same location, not as one explaining the other.