Primary Lymphoid Organs

The thymus and red bone marrow are the two primary lymphoid organs that are sites for lymphocyte production and maturation.

Red bone marrow is primarily located in the axial skeleton, girdles, and proximal epiphyses of long bones.

 Both B and T lymphocytes originate in the red bone marrow.

Here, the cytoplasmic extensions of stromal cells wrap around the developing B-cells, secreting interleukin-7, which promotes B-cell differentiation.

Once mature, the B-lymphocytes enter the bloodstream, moving to secondary lymphoid tissues, such as the spleen and lymph nodes.

Immature or pre-T cells formed in the red bone marrow migrate to the thymus, a two-lobed organ between the sternum and aorta in the mediastinum.

The thymic lobes are further divided into lobules by trabeculae, each with an outer cortex and inner medulla.

T-cell maturation begins in the cortex, assisted by the epithelial cells that secrete hormones called thymosins, aiding in T-cell proliferation.

These maturing T-cells enter the medulla, from where blood vessels distribute them to secondary lymphoid tissues.