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Atypical gamma immunoglobulins characterized by their irreversible heat denaturation at 56-degrees C. Pyroprecipitation is inhibited at pH below 3 and above 9. The presence of pyroglobulins in the serum is the cause of pyroglobulinemia. They are frequently present in multiple myeloma and the pyroglobulin precipitate binds complement, reacts with rheumatoid factor, produces passive cutaneous anaphylaxis, generalized passive anaphylaxis and passive Arthus-type phenomena.