The human lymphoid system

Shown in red are the primary organs (bone marrow and thymus) and shown in blue are secondary organs and tissues (including the spleen, lymph nodes, and Peyer's patches).

These structurally and functionally diverse lymphoid organs and tissues are interconnected by the blood vessels (not shown) and lymphatic vessels (purple) through which lymphocytes circulate. Only one bone is shown, but all major bones contain marrow and thus are part of the lymphoid system.