Against the dark silhouette of a human form, two of the systems organs glow bright: the thymus, a small, two lobed organ just behind the breastbone, and the soft, gelatinous tissue of the marrow deep within our long bones. In these primary lymphoid organs, lymphocytes grow and develop.
Also glowing are the secondary lymphoid organs, the sites where lymphocytes are stored and where some immune responses take place. These include the spleen, an organ in the upper abdomen that filters blood, and the lymph nodes, pulpy clumps of tissue as well as the tonsils, adenoids, appendix, and the Peyer’s patches, bits of lymphoid tissue embedded in the walls of the small intestine.
Amazing isn’t it?