In primary lymphoid organs such as bone marrow and thymus, immature lymphocytes develop into antigen-sensitive lymphocytes. After this maturation process, these lymphocytes move to secondary lymphoid organs, where they can interact with antigens. Secondary lymphoid organs include the spleen, lymph nodes, and Peyer's patches of the small intestine, as well as the appendix. These are critical sites for the interaction between lymphocytes and antigens.