The bone marrow-derived lymphocytes are known as B lymphocytes or B cells. Plasma cells are derived from mature B cells. Both B cells and plasma cells synthesize and secrete immunoglobulin.
B lymphocytes or B cells are so designated because the bursa of Fabricius, a lymphoid organ located close to the caudal end of the gut in birds, plays a key role in their differentiation. A mammalian equivalent of the bursa is yet to be found. Here the early stages of maturation of these lymphocytes occur in the bone marrow.
Origin of B cells: The clonal selection theory explains the ori-gin of antibody formation. According to this postulation, each immunologically competent B cell possesses receptor for either IgM or IgD that can combine with one antigen or closely related antigens. After binding of the antigen, the B cell is activated to proliferate and form a clone of cells. Selected B cells are trans-formed to plasma cells that secrete antibodies specific for the antigen. Plasma cells synthesize the immunoglobulin with the same antigenic specificity as those carried by activated B cells. The same clonal selection also occurs with T cells.
B cell precursors, during embryogenesis, first proliferate and develop in the fetal liver. From there, they migrate to the bone marrow, the main site of B-cell maturation in the adults. Unlike T cells, they do not require the thymus for maturation. The Pre-B cells have only m heavy chains in the cytoplasm but do not have surface immunoglobulins and light chains. Pre-B cells are found in the bone marrow, while B cells are found in the circulation. B cells mature in two phases:
· Antigen-independent phase, which consists of stem cells and pre-B cells
· Antigen-dependent phase, which consists of the cells, such as activated B cells and plasma cells that proliferate on inter-actions of antigen with B cells.
B cells possess surface IgM, which acts as a receptor for antigen. Some B cells may also carry on their surface IgD as receptor for the antigen. There are many other molecules expressed on the surface of the B cells, which serve different functions. A few of them are B220, class II MHC molecules, CR1 and CR2, CD40, etc.
Activation of B cells: Activation of B cells to produce the fullrange of antibodies first requires recognition of the epitope by the T-cell-antigen receptor and the production of IL-4 and IL-5 by the helper T cells. In addition, it also requires other costimu-latory interactions of CD28 on the T cells with B7 on the B cells. The CD28–B7 interaction is essential to produce IL-2. It also includes CD40L on the T cells, which must interact with CD40 on the B cells. The CD40L–CD40 interaction is essential for class switching from IgM to IgG and for switching between other immunoglobulin classes to take place.
Effector functions of B cells: Production of many plasmacells is the end result of activation of B cells. The plasma cells in turn produce large amounts of immunoglobulins specific for the epitope of the antigen. Some activated B cells also produce memory cells, which remain in a stage of quiescence for months or years. Most memory B cells have surface IgG that acts as the antigen receptor, but some even have surface IgM. These quiescent memory cells are activated rapidly on reexposure to antigen. Memory T cells produce interleukins that facilitate antibody production by the memory B cells. The presence of these cells is responsible for the rapid appearance of antibody in the secondary immune responses.