What organ of the lymphatic system is located in the upper left quadrant of the abdomen?

The spleen is the correct answer because it is specifically located in the upper left quadrant of the abdomen. This organ plays several crucial roles in the lymphatic and immune systems. The spleen filters the blood, removing old or damaged red blood cells and pathogens. It also stores white blood cells and platelets, contributing to the immune response.

The other options are not situated in the upper left quadrant of the abdomen. The thymus is located in the upper chest behind the sternum, involved primarily in the maturation of T-cells. Tonsils are lymphoid tissues located in the throat and are part of the immune response system but do not reside within the abdominal cavity. Lymph nodes are distributed throughout the body, including in the abdomen, but they are not specifically located in the upper left quadrant as a singular organ like the spleen.