Final answer:
The spleen functions as a blood reservoir, regulates red blood cell numbers, produces antibodies, and lymphocytes. It filters the blood and initiates immune responses to pathogens.
Step-by-step explanation:
The organ that serves as a reservoir of red blood cells, regulates their numbers, produces antibodies and lymphocytes is the spleen. The spleen is a lymphatic organ located in the upper left quadrant of the abdomen. It is responsible for several key functions in the circulatory and lymphatic systems including filtering the blood, removing unwanted red blood cells, detecting viruses and bacteria, triggering the release of pathogen-fighting cells, and also functioning as a location for immune responses to blood-borne pathogens. The spleen contains two types of tissue, red pulp and white pulp. The red pulp stores blood and removes damaged red blood cells while the white pulp is rich in lymphocytes that remove antigen-coated bacteria from the blood.