Why did many native-born Americans see immigrants as a threat?

"Nativists, as they were called, were profoundly hostile to immigration. They thought that immigrants constituted a threat to the way of life they valued and cherished. To them, immigrants were strange people speaking strange languages which they could not understand. They came from different cultures with radically different political and legal systems. Native-born Americans were afraid that the newcomers would radically change the very structure and nature of American civic institutions."