These are proteins that precipitate from blood at low temperatures. A precipitate from serum that forms overnight at 4°C and dissolves at 37°C is called a cryoglobulin.
Cryoglobulins may be divided into three classes. Type I are monoclonal immunoglobulins and are usually associated with lymphoproliferative disorders. Type II are mixtures of a monoclonal IgM and polyclonal IgG, and are associated with macroglobulinemia and chronic active hepatitis. Type III are mixtures of polyclonal IgM and polyclonal IgG. These are found in a wide variety of disorders.
A high percentage of patients with cryoglobulinemia have clinical symptoms, and of these the most common are vascular (ie, purpura and digital necrosis). Raynaud phenomenon is also common.
Patients with SLE who are rheumatoid factor-negative but cryoglobulin-positive are more likely to develop renal disease than those who are rheumatoid factor-positive and cryoglobulin-negative.