4 weeks before
The thymus is a little organ that sits behind the collarbone, but it performs an important job. The thymus is where essential immune cells called T cells are made and trained to go out into the body and fight infections and cancer.
A peculiar feature of the thymus is that it disappears as we get older. The thymus starts deteriorating after birth but the process speeds up after puberty and, by age 65, we are basically unable to make new T cells.
As the organ shrinks, the T cell areas are replaced with fatty tissues, in a process called involution. As it is replaced by fatty tissues , the thymus can't perform the same immune functions.