What is one of the key functions of the spleen?

The spleen plays a crucial role in the immune system and the circulatory system, making its primary function to filter blood and help fight infections. It acts as a blood reservoir, allowing it to store and recycle red blood cells and platelets. As blood passes through the spleen, it removes old or damaged red blood cells and pathogens.

Additionally, the spleen is involved in the production and activation of lymphocytes, which are essential for the body's immune responses against infections. This function underscores its importance in maintaining the body's defense against pathogens, thereby highlighting its role beyond just blood filtration.

In contrast, the other options pertain to functions that are managed by different organs in the body, such as bile storage in the gallbladder, blood sugar regulation primarily by the pancreas, and red blood cell production, which is largely the responsibility of the bone marrow.