Spleens are one of the more vulnerable organs. They're prone to rupture during trauma, and if ruptured, can easily cause a person to bleed out. In many motorcyclist accidents, a ruptured spleen can be the cause of death.

Spleens are essentially large bags of blood and immunological pulp, and the reason they evolved this way is because they act as a filter against bloodborne pathogens. They host a diverse but specialized collection of immune cells, such as macrophages, and have a truly unique and interesting role in overall health.

You should read up on the thymus next. That one is super important and is also implicated in aging.