B Cells that Produce IgM Antibodies
B cells that produce IgM antibodies play a significant role in immune responses. These cells include B-1 cells and B-2 cells, which produce different types of IgM antibodies and function in different mechanisms with target organs. It is known that most serum IgM antibodies contain natural antibodies with low affinity for antigens, originating from B-1 cells (innate B cells). However, some high-affinity serum IgM antibodies are produced by B-2 cells (conventional B cells).
Introduction to B-1 Cells
B-1 cells are a type of innate B lymphocytes that can produce natural IgM antibodies. The IgM antibodies produced by B-1 cells have a broad antigen recognition ability and can provide initial defense against pathogens and infections. B-1 cells are mainly present in body cavities and intestinal tissues, accounting for about 15% of peripheral blood B lymphocytes. B-1 cell development primarily occurs in the liver during the fetal stage and can also occur in the bone marrow during adulthood. Environmental antigens and cytokine signals are crucial for the survival and function of B-1 cells. Thymic stromal lymphopoietin and IL-5 and other cytokines can prolong the survival of B-1 cells. Toll-like receptors can also activate B-1 cells to produce immunoglobulins. Based on the expression of CD5 molecules, B-1 cells can be divided into two categories: B-1a and B-1b. B-1a cells express CD5 molecules to negatively regulate B cell receptor signaling to prevent overactivation. B-1a cells can produce natural IgM antibodies, neutralize pathogens, and activate immune responses. B-1b cells do not express CD5 and can produce antigen-specific IgM, as well as undergo isotype switching to produce IgG and IgA. In addition, B-1 cells can secrete anti-inflammatory cytokines such as IL-10 to suppress excessive inflammatory reactions. T cells activated by B-1 cells can differentiate into regulatory T cells, maintaining humoral immune tolerance. After phagocytosis of self-antigens or pathogens, B-1 cells can directly stimulate natural killer cells and dendritic cells to activate innate immunity.
Mechanisms of differentiation of B-1a and B-1b cells. (Matsuda, 2020)
IgM Antibodies Produced by B-1 Cells
IgM antibodies produced by B-1 cells are involved in the pathology of various diseases by activating complement and binding to Fcμ receptors. Natural IgM antibodies neutralize toxins and viruses, remove apoptotic cells, and protect tissues from autoimmune attacks; they also promote phagocytosis of pathogens and activate lymphocytes. In transplantation immunization, IgM antibodies produced by B-1 cells may protect the transplanted organs through the above pathways, but the exact mechanism remains to be investigated. Collecting B-1 cells and inducing their differentiation into antibody-producing cells, or isolating the IgM antibodies produced by B-1 cells, will provide a new strategy for transplantation immunotherapy. In conclusion, B-1 cells and the IgM antibodies they produce play an important role in innate immunity and have a potential impact on the pathologic process of various diseases. In-depth studies will help to develop diagnostic and therapeutic tools.
In conclusion, B-1 cells are a specialized subpopulation of B cells that have a unique mechanism for producing IgM antibodies. However, there are a number of issues that need to be addressed, such as the difficulty of differentiating B-1 cells into antibody-producing cells in vitro, since autoantigens and environmental factors play an important role in the survival and growth of these cells in vivo. Therefore, clinical strategies are urgently needed to maintain the supply of B-1 cell-derived IgM antibodies. In addition, if it is possible to categorize natural IgM antibodies into B-1a and B-1b cell-derived antibodies and to explore their role in target organs, it may make an important contribution to the development of more effective therapeutic approaches.
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- Matsuda Y; et al. Characteristics of Immunoglobulin M Type Antibodies of Different Origins from the Immunologic and Clinical Viewpoints and Their Application in Controlling Antibody-Mediated Allograft Rejection. Pathogens. 2020, 10(1): 4.
- Haas K.M.; et al. B-1a and B-1b cells exhibit distinct developmental requirements and have unique functional roles in innate and adaptive immunity to S. pneumoniae. Immunity. 2005, 23: 7-18.
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