Primary Lymphoid organs



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Main function of bone Marrow.

  1. It is the site of origin of all T and B cells, Phagocytes, platelets, erythrocytes and

other leukocytes in adults. All the cells of the blood, including lymphocytes, are produced from hemopoiteic stem cells (HSC) to give rise all elements of the blood.

  1. In addition to hematopoiesis. Bone marrow is the site of removal of aged and defective erythrocyte.

3. Site of differentiation and maturation of B-lymphocytes. In bone marrow, the B-cells develop their receptors during different stages in bone marrow. The mature B-cell with IgM+/IgD receptors leaves the bone marrow and circulates in the blood , but it is considered a "primary" B-cell until it "switches" its membrane immunoglobulin to another isotype. In addition B-cells in bone marrow undergo gene rearrangement, where the germline genes encoding the variable and hypervariable regions of the immunoglobulin light and heavy chains take place in the bone marrow. In addition self-reactive B-cells may be deleted in bone marrow. The deletion of B-cells in bone marrow involves the B-cell clones with a high affinity for self –antigen during the phase of transient from pro-B to mature B-cells, and this process is known as negative selection. This process will maintain the self-tolerence and prevent the autoimmune reaction and subsequently prevent the occurrence of autoimmune disease



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