Charles J. “C.J.” Nessralla IV was a 24-year-old Northeastern graduate bound for medical school. He roomed on one side of Tremont Street — the Northeastern side — in the South End.

Manny “Alex” Peguero was a 15-year-old immigrant growing up without his parents on the other side of Tremont — the more dangerous side — in a crowded apartment in the Lenox Street public housing project.

When Nessralla arrived at Northeastern, his orientation leader stood on the university side of Tremont, gestured toward the housing project, and warned, “Don’t ever cross the street.’’

But Nessralla did, and lives were changed forever.

Six years after their paths first crossed, Manny will wake up Sunday in Nessralla’s childhood home in suburban Avon, an elegant, four-bedroom Colonial on a lushly landscaped lot with a white picket fence and backyard waterfall. There, a Father’s Day breakfast will be cooking, and a boy from the Dominican Republic, whose biological parents long ago vanished from his daily life, will be with the found family that has welcomed him, enjoying a future and a daily routine no longer preoccupied with surviving the city streets.