[2][3] Von Dungern and Hirszfeld examined 348 individuals from 72 families and showed that blood groups A and B did not occur in the offspring unless they were present in at least one of the parents, fulfilling the Mendelian principles of inheritance.

His wife Hanka (1884–1964, born Hanna Kasman), also a physician, became an assistant at the Zurich Children's Clinic under Emil Feer.

In 1914 Hirszfeld was made an academic lecturer on the basis of his work on anaphylaxis and anaphylatoxin and their relationships to coagulation; he was also named "Privatdozent."

Their report was accepted by The Lancet and published in 1919, and it was the first paper showing that blood group frequencies differ between populations.

After the occupation of Poland by the German Army Hirszfeld was dismissed as a "non-Aryan" from the Hygiene Institute but, through the protection of friends, managed to do further scientific work at home until February 1941; it was, however, almost impossible for him to publish.

There he organized anti-epidemic measures and vaccination campaigns against typhus and typhoid, as well as conducting secret medical courses.

[12] Between March and June he and his family fled the ghetto and were able to survive underground through using false names and continually changing their hiding place; his daughter died of tuberculosis in the same year.

He continued his research on blood groups and together with obstetrician prof. Kazimierz Jabłoński, introduced exchange transfusion as a treatment for Hemolytic disease of the newborn, which saved the lives of almost 200 children.

A few months before his death, the Institute of Immunology and Experimental Therapy in Wrocław, now affiliated with the Polish Academy of Sciences and named after him, was created.