The thymus is a specialized immune system organ. The only known function of the thymus is educating and producing T cells. The thymus is a T cell’s college.
The thymus does most of its work early in life. The thymus is largest and most active during neonatal and pre-adolescence, reaching its maximal size during adolescence, typically by the early teens, and gradually declining in size thereafter.
T cell maturation involves learning to distinguish between foreign matter and cells of the self, including the microbiome. This schooling is called tolerance induction, This educational process not well understood.
There are several mechanisms that induce tolerance, which involve a sophisticated complex of sequenced communications and responses that must be learned and passed on to the next generation of immune system cells.
Immature lymphocytes are tested for self-recognition. Cells that bind too strongly to self-antigens, and thus would signal attack on the body’s own cells, are alternately not allowed to mature, edited for other functioning, or deleted.
98% of thymus T cells (thymocytes) fail the tests: either not recognizing enemy microbes or considering friendlies as unfriendly. The remaining 2% survive to serve active duty.
Helper T display on their surface copies of a single protein chain called CD4, and so are sometimes referred to as CD4 T. Killer T display the dimeric (doubled) CD8, hence CD8 T. Immature T cells arrive in the thymus uncommitted to a career as helper or killer. That decision is made during maturation.