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<\/p>\nImmunoglobulin M (IgM) is a central player in the immune system\u2019s early response to infection. Its pentameric structure provides a high-avidity defense mechanism, allowing it to rapidly neutralize pathogens. Yet, despite its critical role, the mechanisms by which IgM exerts downstream functions\u2014particularly through interactions with Fc receptors\u2014remain less explored compared to IgG.<\/p>\n
Among these, the Fc\u03bc receptor (Fc\u03bcR) is an IgM-specific Fc receptor expressed on immune cells including B cells, T cells, and natural killer (NK) subsets. It mediates a range of immunological responses, including B cell survival, early activation, and regulation of antibody-mediated immune communication. To harness the therapeutic potential of IgM, it is crucial to understand how it binds to Fc\u03bcR at the molecular level.<\/p>\n
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Fig.1 The binding sites of Fc\u03bcR and Fc\u03bc-C\u03bc4.1<\/sup><\/span><\/p>\n IgM is composed of five antibody subunits joined by a J chain, forming a complex 900 kDa pentamer. Each subunit includes constant domains C\u03bc1 through C\u03bc4, where the Fc portion comprises C\u03bc2, C\u03bc3, and C\u03bc4. The Fc\u03bcR, on the other hand, is a type I transmembrane protein featuring a 107-amino acid extracellular immunoglobulin-like domain, followed by a 127-amino acid stalk region, a transmembrane region, and a long cytoplasmic tail.<\/p>\n Specificity<\/strong>: Binds exclusively to IgM, unlike pIgR or Fc\u03b1\/\u03bcR that can engage both IgM and IgA.<\/p>\n<\/li>\n Expression Profile<\/strong>: Found predominantly on B cells, T cells, and some NK cells.<\/p>\n<\/li>\n Functional Role<\/strong>: Contributes to tonic BCR signaling, B cell survival, immune tolerance, and modulates T cell responses.<\/p>\n<\/li>\n<\/ul>\n By generating a series of IgM-Fc mutants and evaluating their ability to interact with recombinant Fc\u03bcR, it is possible to map the critical residues responsible for high-affinity binding. These studies provide not only a structural understanding but also enable the design of engineered antibodies with tunable Fc\u03bcR engagement.<\/p>\nStructural Overview: IgM and Fc\u03bcR<\/h2>\n
Key Features of Fc\u03bcR:<\/h3>\n
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Dissecting the Fc\u03bcR Binding Site on IgM<\/h2>\n
Key Mutations and Their Effects:<\/h3>\n
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