B cells are specialized type of lymphocytes that are responsible for the production of antibodies or immunoglobulins that act as effector molecules to stimulate the entire immune system against an invading antigen or pathogen. They are primarily responsible for the development of antibody mediated immunity (AMI) in the body. B cells are produced in the bone marrow. In birds, B cells are produced in the Bursa of Fabricius. Bursa of Fabricius isa lymphoid organ situated near the terminal end of the gut in bird or fowl. They are usually located around the cloacae region of birds. The removal of Bursa of Fabricius from chicken or birds (through a medical procedure known as bursectomy) will impair antibody mediated immune response in the chicken.
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The reason for the impairment of antibody mediated immunity is because the affected birds will no longer secrete B cells which are biologically significant for the production of antibodies and memory B cells during the invasion of pathogens.This also applies to mouse whose thymus is removed through thymectomy. Thymectomy is the medical procedure of removing the thymus through surgery. This procedure exposes the animal to incessant attacks from pathogens especially intracellular parasites. Such mice would find it difficult to produce effector T lymphocytes because the thymus is the organ where T cells mature after their production in the bone marrow. Mice whose thymuses have been surgically removed will generally have an incapacitated cell-mediated immune response especially to intracellular pathogens including viruses which are known to reside within their host cells.
The bone marrow is the site where all the cells of the immune system (e.g., B cells and T lymphocytes) inclusive of blood cells (e.g., red blood cells, white blood cells and platelets) are initially derived from. These important cells of the immune system are derived from haematopoietic stem cells (HSCs) during haematopoiesis in the bone marrow. Haematopoiesisis the process of blood formation in the bone marrow. Stem cells are self-renewing cells found in the bone marrow and which undergo cell division to differentiate and proliferate into other important cells inclusive of the immune system cells and blood cells. HSCs are pluripotent or multipotent in nature because they can differentiate into a wide variety of other cells (e.g., lymphocytes and erythrocytes).B cell maturation is a continuous process that occurs throughout life in the bone marrow of adults. However, the process of B cell maturation in neonates or fetus in utero (i.e., before birth) usually occurs in the fetal bone marrow, yolk sac or fetal liver. And after birth, B cell maturation continues in the bone marrow. After production in the bone marrow, the B cells migrate to secondary lymphoid organs such as the lymph nodes where they differentiate into plasma cells that mediate or stimulate antibody production but only in the appearance of an antigen.
Apart from differentiating into plasma cells for antibody production, the B cells are also responsible for the presentation of antigens to T helper cells which facilitate cell-mediated immunity (CMI) in the host. Thus, B cells can act as both antigen presenting cells (APCs) and synthesizing machinery for antibodies and memory cells. T cells only recognizes antigen molecules presented in association with MHC molecules. Nevertheless, B cells can recognize antigen molecules by itself and without the assistance of MHC molecules, and this is a major difference between B cells and T cells. T cell maturation occurs in the thymus; and its activation and differentiation occurs in the peripheral lymphoid tissue (e.g., spleen) after encountering an antigen presented by MHC molecule(s).
The activation of B cells is driven by the introduction of foreign bodies or antigens into the body. And once activated; B cells undergo proliferation and differentiation into numerous plasma cells for the production of abundant antibodies with unique specificity for antigens. During differentiation or proliferation, the B cells mainly develop into two types of cells viz: memory cells and plasma cells. While plasma cells are largely responsible for the production of antibodies and other effector molecules, the memory cells or memory B cells remain in the circulating blood or body fluids after the primary immune response to police the body and eliminate the invading antigen should the body become exposed to a previously exposed antigen or foreign body the second time (i.e., in the future). This usually happens during a secondary immune response in which the immune system mounts a rapid attack against the invading pathogen – should it appear the second time. The stages involved in the production of immunocompetent B cells in the bone marrow are a complex set of biological process (Figure 1).
The stages involved in the production of immunocompetent B cells in the bone marrow are:
- Maturation: At maturation stage, immunocompetent B cells are produced in the bone marrow by competent progenitor stem cells.
- Activation: Once produced, the immunocompetent B cells become activated following their interaction with antigens or invasion of antigens.
- Differentiation: After the interaction of the immunocompetent B cells with the antigen(s), the activated B lymphocytes differentiate into plasma and memory B cells.
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