Cryoglobulins are abnormal immunoglobulins which form complexes and precipitate out of serum at low temperatures and re-solubilise on warming. They can cause problems by causing the blood to be abnormally thick which increases the risk of blood clots forming in the brain (stroke), eyes, and heart. Cryoglobulins are also associated with inflammation of blood vessels (vasculitis) which increases the risk of blockage of arteries. Three types of cryoglobulins are recognized: single monoclonal immunoglobulin (usually IgG, or IgM), mixed monoclonal/polyclonal immunoglobulins, and mixed polyclonal immunoglobulins. IgM cryoglobulins tend to precipitate at lower temperature than do IgG cryoglobulins. Cryoglobulins are a key part of a condition called essential mixed cryoglobulinemia, and they can also accompany other diseases such as hepatitis C, multiple myeloma, dermatomyositis, lymphoma, systemic lupus erythromatosus, and rheumatoid arthritis.