… has died. Both immunologists, they worked together at the National Institutes of Health in the 1960s (UD‘s father spent his whole career at NIH).
I can’t find papers they co-authored, but they cited each other a lot. Benacerraf’s name, for instance, shows up as the last reference in this paper (UD‘s father is Herbert J. Rapp), “Heterogeneity of Rabbit IgM Antibody as Detected by C′1a Fixation.”