B Cell Development and Proliferation

B Cell Development

  • In the embryonic stage, B cells develop in the yolk sac.
  • In the foetus, it moves to the liver. Later it goes to the bone marrow.
  • In the adult, bone marrow is the major site of B cell development. .
  • In Mammals, bone marrow is the site of B cells development.
  • In Birds, bursa of Fabricius is the major site of B cell development.
  • B cells develop from haematopoietic stem cells.
  • The haematopoietic stem cells are present in the medulla of bone marrow.
  • They are multipotent stem cells.
  • They have the ability for self renewal.
  • They give rise to blood cell types including B cells and T cells.
  • They give rise to two cell lines namely myeloid lineages and lymphoid Lineages.
  • The myeloid lineage gives rise to
  • Erythrocytes
  • Platelets
  • Monocytes
  • Macrophages
  • Neutrophils
  • Basophils
  • Eosinophils
  • Dendritic cells, etc.
  • The lymphoid lineage gives rise to lymphocytes such as B cells, T Cells, NK cells, etc.
  • They resemble lymphocytes in morphology. They are rounded with a large rounded nucleus. The cytoplasm is thin.

Haematopoiesis

  • Haematopoiesis (haemo-poiesis) is the blood cell development. B cells develop by the process of haematopoiesis. The development of lymphocytes from haematopoietic stem cells is called Lymphopoiesis.

Site of Haematopoiesis

  • In the developing embryos haematopoiesis starts in the yolk sac as blood Islands.
  • As development progresses, blood develops in the spleen, liver and lymph nodes.
  • When bone marrow develops, the entire process of haematopoiesis occurs here.
  • In children haematopoiesis occurs in the bone marrow of long bones such as femur and tibia.
  • In the adults, it mainly occurs in the pelvis, cranium, vertebrae and sternum.
  • The B cells are derived from haematopoietic stem cells. Some of the haematopoietic stem cells turn into pro-B cells (progenitor B cells). The pro-B cells differentiate into pre-B cells (Precursor B cells).
  • Pre-B cells enter the bursa of Fabricius and develop into B lymphoblasts.
  • In mammals this change occurs in the bone marrow itself.
  • The B lymphoblast changes into immature B cell which has IgM molecule on the surface.
  • The immature B cell changes into mature B cell which has IgD molecules on the surface in addition to IgM.
  • The immature B lymphocytes move into the lymph nodes and spleen from the bursa or bone marrow. Here the B lymphocyte is put into action when it is exposed to antigen.
  • When there is proper antigenic stimulation (viruses, bacterial infection and Toxins), the B cells divide rapidly and form two types of daughter cells, namely plasma cells and B memory cells (secondary B cells).
  • The plasma cells secrete antibodies in response to specific antigen.
  • The memory cells move more actively from blood to lymph.
  • They survive For a longer time and they respond quickly and effectively during the subsequent similar antigenic stimulation. They produce antibodies and thus bring about secondary immune response.

B Cell Proliferation

  • The multiplication of B cells is called B cell proliferation.
  • It occurs in the bone marrow. On each day, 5 × 107 B cells are proliferated. But 5 x 10 cells die i.e., only 10% survive.
  • B cells develop from haematopoietic stem cells. The B cells are produced through haematopoiesis.
  • The formation of B cells is called lymphopoiesis.
  • The haemopoietic stem cells divide into two lineages, namely myeloid Lineage and lymphoid lineage.
  • The myeloid lineage proliferates into erythrocytes, platelets, monocytes, macrophages, neutrophils, basophils eosinophils, dendritic cells.
  • The lymphoid lineage proliferates into two groups namely natural killer cells and lymphocytes.
  • The lymphocytes proliferate into B lymphocytes and T lymphocytes.
  • The proliferation of B cells involves the following steps:
  • The B cells are derived from haematopoietic stem cells. Some of the haematopoietic stem cells turn into pro-B cells(progenitor B cells).
  • The pro-B cells differentiate into pre-B cells (Precursor B cells).
  • Bone marrow stromal cells are necessary to supply correct environment for pro-B cell to grow.
  • B cells initially need direct contact with stromal cells.
  • The direct contact is brought about by cell adhesion molecules such as CD41 present on the pro-B cell and hyalurnic acid present on the stromal cell.
  • After initial contact, the C-kit, of B cell interacts with stem cell factor of stromal cell.
  • This activates C-kit. The pro-B cell begins to divide and proliferates into the next stage cells called pre-B cells.
  • The pre-B cells express IL-7 receptor on their surface.
  • The stromal cells secrete IL-7.
  • IL-7 binds to the IL-7 receptor present on the surface of pre-B cells.
  • The IL-7 releases the pre-B cell from cell adhesion molecules and the pre-B Cell is detached from stromal cells.
  • Pre-B cells enter the bursa of Fabricius and develop into B lymphoblasts.
  • In mammals this change occurs in the bone marrow itself. The B lymphoblast changes into immature B cell which has IgM molecule on the surface.
  • The immature B lymphocytes move into the lymph nodes and spleen from the bursa or bone marrow.
  • In the secondary lymphoid organs the immature B cell changes into mature B cell which has IgD molecules on the surface in addition to IgM.
  • In the secondary lymphoid organs, the B-lymphocyte is put into action when It is exposed to antigen.
  • When there is proper antigenic stimulation (viruses, bacterial infection and toxins), the B cells divide rapidly and form a clone of B cells. These cells develop into two types of cells, namely plasma cells and B memory cells (secondary B Cells). The plasma cells secrete antibodies in response to specific antigen.
  • The memory cells move more actively from blood to lymph. They survive for a longer time and they respond quickly and effectively during the subsequent similar antigenic stimulation. They produce antibodies and thus bring about secondary immune response.
  • The immature B cells do not produce antibodies.
  • The various steps involved in the proliferation of haematopoietic stem cells into immature B lymphocyte are independent of antigens. Hence they are antigen independent processes.
  • The proliferation of immature B lymphocytes into plasma cells and B memory cells is dependent on antigen. So this process is antigen dependent process.

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