All the following are Secondary lymphoid organs except:

Ans: C (Bone marrow) Ref: Grays Anatomy 40th Ed pg . 127Explanation:Organs of the Immune System:The immune system is made up of many different organs and tissues dispersed throughout the body.PRIMARY lymphoid organs are the sites of lymphocyte hirth, maturation, in an Antigen independent fashion.SECONDARY lymphoid organs are the sites of mature lymphocyte selection and expansion in an Antigen dependent fashion.Primary Lymphoid Tissues:Generate and/or mature and educate cells of the immune systemMaturation and central selectionDiversity created in an Antigen independent fashionBone Marrow---------B cellsThymus-----------T cellsSecondary Lymphoid Tissues:Site where mature, immunocompetent lymphocytes are exposed to AntigenAg is collected and brought to tissue and lymphocytes are exposed to Antigen.Lymph nodes-- Antigen from intracellular tissue fluidsSpleen--blood-borne AntigenMALT--Antigen from mucosal surfaces. Includes Pever's patches, tonsils, and adenoids