Step 1: Primary Lymphoid Organ: Bone marrow is where all blood cells (including B-lymphocytes) are produced and where B-cells mature. The Thymus is where T-lymphocytes mature.
Step 2: Secondary Lymphoid Organ: The Spleen acts as a filter for the blood, trapping blood-borne microorganisms. Lymph nodes trap microorganisms that enter the lymph and tissue fluid.
Step 3: Other secondary organs include Tonsils and Peyer's patches of the small intestine.