Sjögren's syndrome, mixed cryoglobulinaemia and the monoclonal gammopathies.

Sjögren's syndrome (SS) represents a pathological model of the evolution from polyclonal B lymphocyte activation to oligo-monoclonal B cell expansion, which may culminate in the development of a true lymphoproliferative disease. The different phases of this process are usually marked by the appearance of type II mixed cryoglobulins in which the monoclonal component is commonly represented by an IgM with rheumatoid factor activity. A similar mechanism exists in mixed cryoglobulinaemia (MC), a different pathological entity in which hepatitis C virus infection has been demonstrated to play an important etiopathogenetic role. Although there are significant differences between SS and MC, they also share many clinical and immunological characteristics, which suggest that common pathogenetic mechanisms may underlie both disorders.