The spleen is part of a few systems! It filters blood and breaks down dead red blood cells to be recycles or excreted as part of the circulatory system, and essentially does the same for white blood cells as part of the immune/lymphatic system. The parts of the spleen that do its job are pretty easy to remember: red pulp filters red blood cells and many phagocytes, while white pulp filters lymphocytes, which are members of white blood cells. The spleen is also home to a reservoir of blood in case of emergencies such as a hemorrhage, and a reservoir of monocytes for emergency immunity.
In short, it's a very important organ for many systems! However, if removed, all is not lost. In the case of a splenectomy, the liver will pick up the slack and do what the spleen would have done. However, the reverse is not true! You can live without a spleen, but you can NOT live without a liver!