Name four secondary lymphoid tissues/organs.

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Secondary lymphoid tissues are where mature lymphocytes encounter antigens and coordinate immune responses, as opposed to primary lymphoid organs where lymphocytes develop. The four structures that fit this role are lymph nodes, spleen, tonsils, and mucosa-associated lymphoid tissue (MALT). Lymph nodes trap and present antigens in a setting that activates B and T cells. The spleen filters the blood and mounts responses to blood-borne pathogens. Tonsils protect the pharyngeal gateway and sample inhaled or ingested microbes. MALT lines mucosal surfaces (such as the gut and respiratory tract) and provides a local immune surveillance network. Therefore, the set comprising tonsils, MALT, lymph nodes, and spleen is the correct collection of secondary lymphoid tissues. The other options include primary lymphoid organs like the thymus and red bone marrow, which are sites of lymphocyte development rather than peripheral immune activation.