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    The humoral immune response: joint teamwork of B cells and antibodies

    The BCR repertoire is defined as the set of unique BCRs found in the B cell lineage cells in a given individual, from pro-B cell to terminally differentiated plasma cells (B2 cell lineage), or B1 cells (B1 cell lineage). An adult human is estimated to maintain 108–1010 distinct BCRs at any given time (total number of B cells: 1011–1012). Correspondingly, the antibody repertoire is defined as the entirety of sequence-distinct secreted antibody proteins in a given individual (its size in terms of unique clonal lineage is unknown and is a major focus of discussion). Thus, by definition, the antibody repertoire is a subset of the BCR repertoire and varies as a function of the dynamics of the B cell repertoire and antibody half-life .

    ASCs include plasmablasts and plasma cells, which belong to the late stages of B cell ontogeny, as well as 'natural antibody'-producing cells. ASCs synthesize and secrete several thousand antibodies per second. Secretion rates may vary across 3–4 orders of magnitude . The antibody repertoire is mainly divided into the mucosal compartment (e.g., gut, lung) and the systemic (serum) compartment. In the mucosal compartment, IgA (mostly IgA2) is generally the most abundant antibody isotype. However, IgA is somewhat less abundant than IgG in urine, bile, and in genital and bronchoalveolar secretions. IgD can be detected in nasal, salivary, lacrimal, and bronchoalveolar secretions, whereas IgE is measurable in nasal, bronchoalveolar, and intestinal secretions, at least when allergy is present. The serum of healthy human donors predominantly contains three antibody isotypes: IgG (~85%), IgA (7–15%, mostly IgA1), and IgM (~5%), and IgD and IgE are only present at low concentrations (~1%). The half-life of antibody molecules in serum is less than 3 weeks.

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