Final answer:
The spleen filters foreign particles from the circulating blood and plays a vital role in the immune response to pathogens.
Step-by-step explanation:
Foreign particles circulating in the blood are filtered by the spleen. The spleen serves a critical role in the circulatory and lymphatic systems by filtering blood and trapping foreign particles, such as bacteria and viruses. It contains red pulp and white pulp; the red pulp removes damaged red blood cells and stores new ones, while the white pulp is rich in lymphocytes that remove antigen-coated bacteria from the blood. This filtering process helps maintain a microorganism-free environment in the blood. Additionally, the spleen is a site where antigen-presenting cells (APCs) communicate with lymphocytes and where antibodies are synthesized and secreted by activated plasma cells, providing a strong immune response to blood-borne pathogens.