Final answer:
The spleen in a dog or cat is located on the left side of the abdomen just caudal to the stomach and is a key component of the lymphatic system, involved in filtering blood and mounting immune responses.
Step-by-step explanation:
In a dog or cat, the spleen is located on the left side of the abdomen just caudal to the stomach. The spleen serves as a major secondary lymphoid organ that is approximately 12 cm long. It is attached to the lateral border of the stomach via the gastrosplenic ligament, lying near the left kidney and below the diaphragm. As part of the lymphatic system, the spleen is heavily involved in filtering pathogens and worn-out red blood cells from the blood, with lymphocytes within it destroying any pathogens that are filtered out. The spleen's extensive vascularization makes it a key site for immune system responses against blood-borne pathogens.