Spleen

The spleen is similar in structure to a large lymph node; it acts primarily as a blood filter. The spleen is brownish in color and is located in the left upper quadrant of the abdomen.

The spleen plays important roles in regard to red blood cells (also referred to as erythrocytes) and the immune system. 

The spleen synthesizes antibodies in its white pulp and removes antibody-coated bacteria and antibody-coated blood cells by way of blood and lymph node circulation.  The spleen is a center of activity of the mononuclear phagocyte system and can be considered analogous to a large lymph node, as its absence causes a predisposition to certain infections.

The spleen is unique in respect to its development within the gut. While most of the gut viscera are endodermally derived (with the exception of the neural-crest derived suprarenal gland), the spleen is derived from mesenchymal tissue. Specifically, the spleen forms within, and from, the dorsal mesentery. However, it still shares the same blood supply—the celiac trunk—as the foregut organs.