Final answer:
Secondary lymphoid organs, including lymph nodes, contain B cells, T cells, lymphoid follicles, and APCs. These organs play a crucial role in filtering lymph and activating lymphocytes in response to pathogens.
Step-by-step explanation:
Secondary lymphoid organs, such as lymph nodes, contain B cells, T cells, lymphoid follicles, and APCs. These organs play a crucial role in the immune response by filtering lymph and activating lymphocytes in the presence of pathogens. For example, lymph nodes contain specialized areas called germinal centers that are rich in B and T cells, while lymphoid follicles are sites of lymphocyte formation. Additionally, APCs in lymph nodes capture and process antigens, informing nearby lymphocytes about potential threats.