Because they were so important to the rural labor force, immigrants and immigration issues come up frequently in the Farm, Field, and Fireside Collection. Although there are certainly fears about the "type" or "class" of immigrant arriving in the country, farmers were much more supportive of immigration than their native-born counterparts in large urban centers, like Chicago. In fact, there are many articles in the collection about the need to attract "foreingers" away from the cities and to the countryside, where they could be put to work for very little pay. This is one reason why many farmers, though not all, were opposed to immigration laws, which would limit the flow of immigrants into the country, and consequently the availability of cheap labor. The documents you find will take up both sides of the immigration debate.
The kind of documents you will find include, but are not limited to:
To do a general database search, begin with simple keyword terms, like "immigrant" or "immigration." To narrow your results, use Boolean logic to combine terms. By searching for "aliens labor
Also, be sure to use the language of the time period you're researching. For example, keyword searches like "alien" or "foreigner" might bring back results that "immigrant" missed. Of course, you won't always be familiar with contemporary language, which is why it's helpful to keep a record of vocabulary as you discover it in the articles you read.
Finally, try variant spellings and variant forms. For example, "immigrant" or "emigrant."