[6][7] In 1962, Ishizaka and his wife Teruko were recruited by Sam Bukantz, medical director of the Children's Asthma Research Institute and Hospital (CARIH), and moved to Denver, Colorado.

[8] He assumed the post of assistant professor of microbiology at the University of Colorado Medical School, as well as chief of immunology of its associated Children's Asthma Research Institute[6] and CARIH.

[6] While at Denver, the Ishizakas discovered the antibody class Immunoglobulin E (IgE) in 1966–1967[10][11] and its interplay with mast cells.

[3] They demonstrated the IgE's critical role in mediating the release of histamine from mast cells.

[13] After retiring in 1996,[14] he returned to Japan and served as honorary director of the Institute of Immunology at Yamagata University.