When a fetus nears the end of the third trimester, a billion lymphocytes (a type of white blood cell) leaves the bone marrow and enters the thymus, a marble-sized organ that sits just above the heart. They become T lymphocytes. The t-cells then reorganize their DNA, something that no other cell in the body does, and produce 10^15 T-cell receptor proteins. These are the proteins that lock onto invading viruses, bacteria, parasites and fungi as part of our immune system response. The number is huge. If you counted that number as seconds on a clock, it would take 500 million years to get to 10^15. Those are the numbers required to defend us against the numerous threats we'll suffer during the course of a lifetime. But first, the T-cell receptors have to pass a test. The thymus disguises its cells as different parts of the body, to test if the T-cell receptors will attack them. If they do, then the thymus moves swiftly to destroy them. If the thymus fails to eradicate these rogue T-cell receptors, then the fetus will have an auto-immune disease. So even before birth, the thymus contains a complete map of our selves. It creates replicas of lung cells, liver cells, heart cells, retina cells, muscle cells, etc. The thymus has the responsibility of carving self from not-self (viruses, bacteria, parasites, fungi). --- Source: [[Lousy Sex]]