Benacerraf, Baruj  ( born October 29, 1920 , Caracas, Venezuela—died August 2, 2011 , Boston, Massachusetts, U.S. )  Venezuelan-born American pathologist and immunologist who shared (with George Snell and Jean Dausset) the 1980 Nobel Prize for Physiology or Medicine for his discovery of genes that regulate immune responses and of the role that some of these genes play in autoimmune diseases.

From the age of five until the outbreak of World War II, Benacerraf lived in Paris. In 1940 he entered Columbia University in New York City, from which he graduated in 1942. He became a naturalized U.S. citizen in 1943, while a student at the Medical College of Virginia in Richmond. After receiving an M.D. in 1945 and interning at Queens General Hospital in New York City, he served (1946–47) in the U.S. Army in 1946–47Medical Corps. Benacerraf then spent a year in immunological research at the Columbia University College of Physicians and Surgeons. He moved on to the French National Center Centre for Scientific Research at the Broussais Hospital in Paris, where he continued to study immunology. In 1956 he joined the faculty of New York University (NYU) School of Medicine. He advanced to professor of pathology in 1960, a position he held until 1968.

At NYU Benacerraf began to study the genetics of the immune system. His experiments led to his development of the concept of immune response (Ir) genes, which control the immune system’s ability to respond to antigens (infectious agents or foreign materials that enter the body). More than 30 Ir genes were subsequently found, and that genetic material was determined to be part of the major histocompatibility complex, a complicated region of DNA involved in immune responsiveness. Benacerraf’s findings also helped elucidate the mechanisms underlying autoimmune diseases, such as multiple sclerosis and rheumatoid arthritis, in which the immune system mistakenly mounts an attack against its own tissues.

In 1968 Benacerraf became chief of the immunology laboratory at the National Institute of Allergy and Infectious Diseases in Bethesda, Maryland. He was appointed From 1970 to 1991 he served as both professor of comparative pathology and chairman of the pathology department at Harvard University Medical School in 1970 and became president . He also was president (1980–91) of the Sidney Farber Cancer Institute (now the Dana-Farber Cancer Institute) in Boston in 1980. Benacerraf was elected to the National Academy of Sciences (1973) and was awarded the National Medal of Science (1990). He published a number of books, including the Textbook of Immunology (1984) and his autobiography, From Caracas to Stockholm (1998).