For most of us, the jargon of modern immunology is incomprehensible. Even immunologists themselves frequently have trouble understanding their colleagues. So the plainspoken Melvin Cohn is a welcome island in a confusing sea of concepts like self/nonself, costimulatory signaling, and sorting the repertoire. And in fact, Cohn, who at 83 is still a fixture at the Salk Institute for Biological Studies in La Jolla, CA, has clarified his ideas to the point that he has come up with a list of discrete immunologic rules. Those rules allowed him to build a computerized version of the entire human immune system. "If you can't make it clear, you don't understand it yourself," says Cohn.

A microbiologist and biochemist by training, Cohn has spent more than half a century investigating the immune system. He views himself not as a theoretician but as a conceptualist, someone who tries to assemble many small theories into a comprehensible big picture. "The theoreticians largely came from physics and mathematics, and they didn't understand immunology," says Cohn, who leads the Conceptual Immunology Group at the Salk Institute. He is well known for elucidating provocative and fiercely debated concepts about the evolution of the immune system and how it operates. "Mel is the smartest, most thorough, most creative living immunologic theoretician," says Polly Matzinger, an immunologist at the National Institute of Allergy and Infectious Diseases.