n. the branch of physics concerned with the study of extremely low temperatures and resulting phenomena. It has diagnostic and therapeutic application, as when certain immunoglobulins associated with a variety of diseases are detected in the blood by reducing the temperature of a blood sample to the point at which the substances separate from the blood serum. Cryogenic methods are also used to freeze tissue samples to be sliced for microscopic examination or to be destroyed, as in cryogenic surgery, which selectively destroys diseased tissue by freezing it.