Klaus Rajewsky (born 12 November 1936 in Frankfurt am Main) is a German immunologist, renowned for his work on B cells. He studied medicine in Frankfurt, Munich and at the Pasteur Institute, Paris. In 1964 he started working at the Institute of Genetics in the University of Cologne, where he became professor for genetics. He researched Hodgkin's disease and the role of B cells within the immune system. He also developed conditional knockout mice based on Cre-Lox recombination.