Spleen, lymph nodes tonsils peyer's patches of small intestine,(vermiform) appendix. (any two) They act as sites for interaction of lymphocytes with the antigen and cause immune response.
Function : spleen - Trap blood -borne microorganisms & worm out RBCs thus filters blood.
Lymph nodes: Trap the microrganisms/antigens (which happen to get in to the lymph and tissue fluid).the trapped antigens activate lymphocytes &cause immune respons.
