Final answer:
The spleen is the organ that serves both as a lymphoid organ and as a reserve for platelets, performing crucial roles in blood filtration, immune response, and blood clotting processes.
Step-by-step explanation:
The organ that not only performs a lymphoid function but also holds a reservoir of platelets is the spleen. The spleen is a secondary lymphoid organ, similar in function to lymph nodes, but much larger and filters blood instead of lymph. Its extensive vascularization and the presence of macrophages and dendritic cells allow it to remove microbes and other materials from the blood, including dying red blood cells.
Furthermore, the spleen serves as a storage site for platelets, which are critical components in the blood clotting process. It also plays a role in mounting immune responses to blood-borne pathogens. Therefore, the spleen is crucial not only for its filtration function but also as an immune system organ and a reservoir for platelets.