Organs which provide the environment for the development, maturation and differentiation of the immature lymphocytes generated in hematopoiesis so that they become committed to a particular antigenic specificity are called as Primary Lymphoid Organ. It includes

  • Thymus
  • Bone Marrow


It is a flat, triangular, bilobed organ situated above the heart and is enclosed within the fibrous capsule. Its each lobe is divided into lobules, which are separated from each other by strands of connective tissue called trabeculate.The function of the thymus is to generate and select a repertoire of T-cells that will protect the body form infection. As thymocites develop T-cell receptors are produced randomly. T-cells with receptors which are capable of recognizing antigen MHC complexes and some which are incapable of recognizing antigen MHC complexes are produced. Stem cells from bone marrow is send to thymus.95% of all thymocytes die by apoptosis in the thymus without even reaching maturity.


It is the site of B-cell origin in mammals. Immature B-cells proliferate and differentiate within bone marrow. The function of bone marrow is to generate and select a repertoire of B-cells that protects the body from infection. During B-cell maturation, a selection process take place within the bone marrow just like thymic selection that ensures the extensive maturation of those B-cells which carry on non self-reactive receptors and eliminates those which possess self –reactive antibody receptors. Bone marrow is not a site for B-cell development in all species. It also produces RBC and Platelets.


It includes organs where the mature lymphocytes settle. It traps the antigen either from lymph or the blood .They include lymph nodes and spleen which is organized into structures called lymphoid follicles.


They are bean shaped structures which contain a reticular network packed with lymphocytes, macrophages and dendritic cells. They are clustered at junctions of the lymphatic vessels it is divided into three regions – cortex, paracortex and the medulla. The space just below the capsule is called as sub capsular sinus. There is a define area of organization for the B and T lymphocytes. Their function is to trap antigens or microorganisms that enter the lymph which results in the activation of lymphocytes and cause the immune response.


Spleen is a large and ovoid lymphoid organ situated in the left abdominal cavity below the pancreas. It traps blood –borne antigens and thus responds to the systematic infections. It consist of a network of sinusoids enriched with macrophages and red blood cells and lymphocytes. Blood-borne antigens as well as lymphocytes enter spleen through the splenic artery. It reaches the marginal zone where antigen is trapped by dendritic cells and carried to peri arteriolar lymphoid sheath (PALS).Dendritic cells along with antigen along with MHC molecules to TH cells and thus activate them which further activate the B-cell. These activated B-cells migrate to primary follicles in the marginal zone .Thus, upon antigenic stimulus, these primary follicles develop into characteristic secondary follicles containing germinal centers with rapidly dividing B-cells and plasma cells.

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