B Lymphocytes and Plasma Cells
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Introduction
B lymphocytes mature within the bone marrow. Mature, naive B cells then migrate to peripheral lymph nodes, where they await B cell activation mediated by dendritic cells and CD4+ effector cells.
Once in the circulation, they express a single, unique immunoglobulin (IgM).
When a B cell encounters an antigen that matches its antibody, it proliferates rapidly and differentiates into memory B cells and effector B cells (Plasma cells).
Memory B cells have a longer life span and express membrane-bound antibody.
Plasma cells produce soluble antibody. Although they live for only a few days, they secrete enormous amounts of antibody, estimated at perhaps 2000 molecules per second.
B Cell Development
B cell development takes place in the bone marrow.
Pro B cells have no surface antibody.
Antibody heavy chain gene rearrangement occurs in one allele, and following trafficking to the membrane it is tested with association with pseudo light chain. The pro B cell has two chances (two alleles) to properly rearrange its heavy chain. Success is signalled by BTK (bruton's tyrosine kinase); mutation of BTK leads to apoptosis and agammaglobulinemia.
Pre B cells express only heavy chain and go through rearrangement of light chain.
Mature B cells express both H and L chains of IgM and migrate to peripheral lymph nodes as naive B cells, where they await activation. Naive B cells die within a few days if they are not activated.
B Cell Activation
After taking up antigen from the periphery, dendritic cells migrate to peripheral lymph nodes where they interact with both CD4+ cells and B cells.
Dendritic cells release antigen near B cells via their dendrites. This antigen, which is protein, not peptide, interacts with B cell surface immunoglobulin (IgM). B cells internalize, process, and express antigen on their MHC Class II receptors, creating a primed B cell.
Following antigen presentation by dendritic cells to CD4+ cells in the lymph node, activated CD4+ cells move to the B cell area of the lymph node.
CD4+ effector cells encounter antigen on B cell MHC II molecules via their T cell receptors, stabilized by B7/CD28. This causes CD4+ cells to secrete B cell growth factor, IL-4.
This concurrent stimulation by dendritic cells and activate CD4+ causes B cell proliferation and differentiation into memory B cells and plasma cells. Cytokine pattern-mediated class switch allows specific immunoglobulins to be produced.
Immunoglobulin Production
Antibodies are produced by B cells. Naive B cells have IgM expressed on their cell surface.
Upon CD4+ effector cell-mediated B cell activation, B cells become memory cells or plasma cells. CD4+ cells also produce signals leading to class switch.
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