Final answer:
The spleen's primary function is to filter microorganisms and other foreign substances from the blood, support immune responses, and remove damaged red blood cells.
Step-by-step explanation:
The primary function of the spleen in the human body is to filter microorganisms and other foreign substances from the blood. This organ also stores blood, removes damaged red blood cells, and plays a significant role in immune responses as it houses B and T cells, macrophages, dendritic cells, and NK cells. The spleen provides an environment in which antigen-presenting cells (APCs) can communicate with lymphocytes, leading to the production and secretion of antibodies. Functionally, the spleen acts as a filter for the blood, similar to how lymph nodes filter lymph.