Final answer:
Foreign antigens in the blood are usually taken to the spleen, a secondary lymphoid organ that filters foreign substances and pathogens. It houses B and T cells, macrophages, dendritic cells, and NK cells and is the site where APCs can communicate with lymphocytes.
Step-by-step explanation:
Foreign antigens in the blood are usually taken to the spleen. The spleen is a secondary lymphoid organ that filters foreign substances and antibody-complexed pathogens from the blood. It houses B and T cells, macrophages, dendritic cells, and NK cells, and is the site where antigen-presenting cells (APCs) that have trapped foreign particles in the blood can communicate with lymphocytes.