Proteins produced by B cells that bind specific antigens
Explanation
Immunoglobulins, often called antibodies, are glycoproteins secreted by plasma cells in response to foreign molecules. Each molecule consists of two heavy and two light chains arranged in a Y-shaped configuration with variable regions at the tips that determine antigen specificity and constant regions that define the immunoglobulin class. The main classes—IgG, IgM, IgA, IgE and IgD—are distinguished by their heavy chain type and biological roles. IgG, the most abundant in serum, crosses the placenta and provides long-term systemic immunity. IgM is produced first during a primary immune response and forms pentamers that efficiently activate complement. IgA is found in mucosal secretions and colostrum, protecting surfaces such as the respiratory and gastrointestinal tracts. IgE binds to mast cells and basophils and mediates allergic reactions and defense against parasites. IgD is primarily a receptor on naive B lymphocytes. Immunoglobulins neutralize toxins, opsonize pathogens for phagocytosis, activate complement cascades and mediate antibody-dependent cellular cytotoxicity. They can also serve as B-cell receptors prior to secretion and form immune complexes that are cleared by the reticuloendothelial system. Genetic recombination of V, D and J gene segments creates a vast repertoire of specificities, and somatic hypermutation and class switching refine affinity and effector functions during an immune response.
Clinical and research examples
Measurement of specific immunoglobulin isotypes guides diagnosis and management. Detection of IgM antibodies to hepatitis A virus or measles indicates recent infection, while IgG suggests past exposure or immunity. Intravenous immunoglobulin therapy, derived from pooled donor plasma, is administered to patients with primary antibody deficiencies or certain autoimmune diseases to provide passive immunity. Secretory IgA in breast milk protects infants against enteric pathogens. Elevated IgE levels are associated with atopic diseases such as asthma and hay fever. In agammaglobulinemia, defective B-cell development results in very low immunoglobulin levels, leading to recurrent bacterial infections. Monoclonal antibodies, engineered versions of immunoglobulins, are used therapeutically to target cancers, autoimmune disorders and viral infections.
Immunoglobulins underpin humoral immunity by recognizing and neutralizing diverse antigens. Their structure and class diversity allow specialized responses at different body sites and stages of infection. Understanding immunoglobulins aids in diagnosing immune disorders and developing targeted therapies.
Related Terms: Antibody, B cell, Immunity, Monoclonal antibody, mmunodeficiency
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