There is something I wanted to share with you guys that I didn't learn today, but was really impressed with while studying. The thymus is a primary lymphoid tissue, meaning it's a site for blood cell development. Specifically, the thymus is responsible for the maturation of T-lymphocytes. You may remember from your high school biology class (it's ok if you don't) that there are "helper" T-cells and "killer" T-cells. These cells are part of the body's adaptive immune system, which recognizes a threat (from bacteria/viruses/fungi) and both attacks it and remembers it for future reference.
The nifty part is that the discerning thymus essentially weeds out both the lazy and homicidal T-cells before releasing the mature, level-headed ones into the body. It does this in two steps: First, it offers up some practice targets that it would like the T-cells to latch onto. If the cells don't bite, they're killed off. The survivors are then released to the second test area. The thymus again offers up targets, but there's a catch: this time the targets are actually pieces of good structures from within the body. If the T-cells go for these, then they are showing autoimmune capabilities and are also killed. This way only the cells that reliably target antigens are allowed to patrol the body. Elegant huh?
I could not for the life of me find a good diagram of this process, so I whipped this up for you:
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