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.
