In the 1800s, there were two main waves of immigration to the United States. These were the “old immigrants” (though of course they were not called that at the time) in the 1840s and the “new immigrants in the late 1800s. There were a few major differences between these two groups, outside of the fact that they came at different times.
The old immigrants mostly came from Germany and Ireland. Many of the Germans were middle class people who had come because they were fleeing the fallout from the attempted revolution in Germany in the late 1840s. This had been a liberal rebellion against monarchy and many democrats fled to the US. The Irish came because of the potato famine.
The new immigrants, by contrast, came from countries in Eastern and Southern Europe. These were Poles and Greeks and Italians. Immigrants had never come to the US from these countries. Many of the immigrants from Eastern Europe were Jews. Many of the immigrants were politically radical. They were anarchists and socialists. Because of these facts, many people in the United States were very concerned about the new immigrants. They thought the new immigrants were dangerous and that they would ruin the US with their undemocratic ways and their “alien” religion.