Abraham H. Foxman, a hidden child of the Holocaust who became the chief combatant against antisemitism in the United States as national director of the Anti-Defamation League for almost three decades, died on May 10 in Manhattan. He was 86.

Mr. Foxman was born near Vilnius. When the Germans invaded the Soviet Union in June 1941 and seized Vilnius, eventually slaughtering 100,000 Jews, the family turned over Avraham to a nanny, Bronkislawa Kurpi. His mother posed as a Christian and Avraham’s aunt. His father, after periods confined to Nazi labor camps, hid out in forests.

For four years the nanny raised Avraham as her son, had him baptized as a catholic and gave him the name Henryk Stanislaw Kurpi. Whenever he passed a church, he would make the sign of the cross, and when he met a priest on the street, he would stop to kiss his hand, the Times said.

The family survived the war, reclaimed 5-year-old Avraham, and returned Avraham to Jewish observance. The family immigrated to the United States in 1950, and Mr. Foxman attended the Yeshiva of Flatbush and stayed through the end of high school. He earned a B.A. in political science from the City College of New York and a law degree from New York University.

Some of my most satisfying moments as director of the ADL were in “witnessing people who did bad things and said vile things turn around and become better people,” he said.

So identified was Mr. Foxman with the battle against antisemitism that in 2016, when campuses of the City University of New York grappled with incidents that many Jews saw as bigoted, Eric Alterman, a professor of English at Brooklyn College, said in an opinion article, “When it comes to antisemitism, if Abe Foxman is not worried, I’m not worried.”