CD5+B cells, recently defined B-1a cells, are the main clonotype in the early B cell repertoire and persist throughout adult life. This subset is committed to the production of antibodies, defined "natural antibodies", that bind a lot of exogenous antigens, isologous and heterologous serum proteins, tissues and/or self antigens, such as hormones, nucleic acids and phospholipids. These antibodies, mostly IgM, but also IgG and IgA, are polyreactive: their role is likely that of first line of defence against invading microorganisms. Natural antibodies produced by CD5+ B cells may be involved in maintenance functions such as removal of dead cells. Because of their ability to bind self antigens, they may be important in the establishment of autoimmune phenomena. An increase of CD5+ B cells has been demonstrated in some autoimmune disorders. The authors examine the functional characteristics and the expansion of this subset in different diseases. In particular they report the increase in active Graves' disease and discuss the role of CD5+ B lymphocytes in the immune imbalance present in this thyropathy.