Multiple Sclerosis (MS) is an inflammatory disease of the Central Nervous System with multifocal areas of demyelination. Although its etiology and pathogenesis remain controversial, several lines of evidence indicate that MS is mediated by a misdirected immune response against one or several myelin proteins. The involvement of diverse leukocyte subsets and their products in MS is still the subject of considerable debate. The emphasis on T cells has derived mainly from the detection of activated T cells in MS plaques and analogies with animal models of MS. Because of these observations cell-mediated immunity has dominated the research field of MS to this day. However, in recent years the role of B cells, plasma cells and immunoglobulins in MS have been re-examined, and current findings indicate that humoral immunity also plays a major role in disease pathogenesis. B cells and their products could exert several potential effects during the course of MS. Firstly, autoantibodies against specific myelin antigens could mediate damage to myelin membranes. Secondly, some studies suggest that natural autoantibodies could enhance remyelination. Thirdly, antibodies directed against myelin components can participate in anti-idiotypic networks, which may regulate the course of MS. Increased intrathecal immunoglobulin synthesis reflected by raised IgG indices and an oligoclonal pattern is the most common abnormality detected in MS patients. The introduction of more sensitive procedures for protein detection has allowed demonstrating oligoclonal bands (OCBs) in up to 95 % of patients with clinically definite MS. Although the presence of OCBs in CSF of MS patients is now well established as a sensitive laboratory test to support the clinical diagnosis, OCBs may be present in other disorders, including those not directly related to infection or abnormal immune response. Nevertheless, the pathogenesis of OCBs in MS is still obscure, and despite extensive research their antigenic target(s) have yet to be established. Therefore, a critical task is to identify the specificity of such target(s), thus providing significant clues about MS etiology. For this purpose, novel molecular immunologic strategies have been recently developed to offer alternative approaches to identify putative antigens recognized by antibodies present in MS patients. The elucidation of the mechanisms and target(s) responsible for the onset of MS has obvious implications for the further development of specific therapies.