Immunoglobulin isotype switching is the process where the Cmy region, initially located downstream of the VDJ region is replaced by another constant region gene. Class switch recombination takes place between switch (S) regions, composed of tandem repeats of short unit sequences, located 5 of each CH gene, except for Cdelta. The class switch machinery works by looping out the sequence between a donor and an acceptor S region and thus allows a single B cell to alter its Ig effector function while retaining the same antigen specificity. This study investigated molecular aspects of immunoglobulin isotype switching in health and disease. In vivo expression of germ-line mRNA in various organs, during B cell development and in immune diseases demonstrated a good correlation between the Ig levels and the expression of germ-line transcription. This findings suggest, that in vivo, a functional correlation exists between germ- line transcriptional activity and isotype switching. Analysis of the molecular basis of IgA deficiency demonstrated a significant decrease in the switch frequency to Salpha1 in these patients. The decrease in the number of switch fragments is consistent with a profound decrease in the Calpha membrane mRNA expression in unstimulated PBMC, as well as in the Calpha mRNA levels and IgA production in PWM-stimulated PBMC. The data indicate that the failure to switch to IgA producing B cells may be an important molecular mechanism in IgA deficiency. Molecular mechanisms leading to IgA production in IgA nephropathy patients showed an increase of both IgA1 and IgA2 subclass specific RNA with a preference for IgA1 and also an increased number of IgA1 and IgA2 producing cells. There was no expression of Ia transcripts in unstimulated PBMC from IgAN patients, but Ialpha RNA could be induced with TGF-beta1. This finding together with the elevated CD40L expression and cytokine production indicate that defect(s) in the immune regulation may play an important role in the pathogenesis of IgA nephropathy. Patients ...