Primary lymphoid organs

Bursa of Fabricius of birds, bone marrow and thymus gland of mammals constitute the primary lymphoid organs involved in the production and early selection of lymphocytes. These lymphocytes become dedicated to a particular antigenic specificity . Only when the lymphocytes mature in the primary lymphoidal organs, they become immunocompetent cells . In mammals, B cell maturation occurs in the bone marrow and T cells maturation occurs in the thymus.

Thymus The thymus is a flat and bilobed organ located behind the sternun, above the heart. Each lobe of the thymus contains numerous lobules, separated from each other by connective tissue called septa. Each lobule is differentiated into two compartments, the outer compartment or outer cortex, is densely packed with immature T cells called thymocytes, whereas the inner compartment or medulla is sparsely populated with mature thymocytes. One of its main secretions is the hormone thymosin .

It stimulates the T cell to become mature and immunocompetent . By the early teens, the thymus begins to atrophy and is replaced by adipose tissue (Fig. . ).

Thus thymus is most active during the neonatal and pre-adolescent periods . Fig. . Primary lymphoid organ - Thymus