Impaired Ig class switch recombination The generation of the antibody repertoire requires two successive steps. The first is antigen- and T-cell-independent; it takes place in the fetal liver and the bone marrow. Immature B lymphocytes rearrange their immunoglobulin-gene variable segments (V segments), diversity segments (D segments) and joining segments (J segments), producing a functionally integrated VDJ segment linked to the mu constant region (C-mu), which determines the primary antibody repertoire composed of IgM antibodies. The second step is antigen- and T-cell-dependent and takes place in secondary lymphoid organs. After encountering antigen, B cells proliferate and form germinal centers. In this unique anatomic formation, two genetic events lead to generation of the secondary antibody repertoire: class-switch recombination (CSR) and somatic hypermutation (SHM). CSR of immunoglobulins occurs by a recombination process between two different switch regions (S regions) located upstream of each C region. Replacement of C-mu by a constant region of another class of immunoglobulin (C-gamma, C-alpha or C-epsilon) ensues and leads to the production of IgG, IgA or IgE. Impaired B-lymphocyte isotype switching UMLS:C1842528 HP:0002959 An impairment of the class-switch recombination process that normally leads B lymphocytes to produce IgG, IgA, or IgE. Abnormal B cell physiology