Immigrants weakening the western economy?

Its not uncommon for people from less well-off countries to travel to the west to work in order to make more money. Some even work here, and send money back to thier native country to support thier families.

What effect does this sort of behaviour have on an economy? Does spending money youve earnt in your own country abroad weaken your home economy?

China is greatest threath to western economy. It maybe cheap to make stuff there, but we should realize that at same time we're strenghtening them.