Final answer:
The main organ located below the diaphragm on the left side of the body is the spleen, which plays a role in the immune system, along with portions of the stomach and the pancreas.
Step-by-step explanation:
The structure that lies below the diaphragm on the left side of the body is mainly the spleen. It is an important organ responsible for filtering blood as part of the immune system. The spleen is also involved in red blood cell and platelet destruction. Adjacent to the spleen, there's the stomach and a portion of the pancreas, both of which are located towards the left side beneath the diaphragm. The pancreas has both endocrine and exocrine functions, including regulating blood sugar levels and aiding in digestion.