Those who are against immigration because they want to "protect their fellow Americans" should first figure out which Americans they are talking about, certainly not those Americans who want to employ immigrants, or those Americans that want to use the services of immigrants, or those Americans who want to buy products made by immigrants.
The opponents of immigration often point to the "other side of the coin". These critics often claim that immigration leads to increases in crime rates, fewer jobs for the home population, and more burden put on services such as schools, health care and public housing. These claims are, for the most part, mistaken.
First, it is important to differentiate between (1) privately arranged immigration and (2) subsidized immigration. The first originates from private arrangements between the home population and immigrants, and the latter is when the government uses tax money to fund immigration programs. Case (2), in fact, may be labeled as forced or haphazard immigration because the government is using the money taken from the home population to invite people into the country even though there are no actual guarantees that any resident of the home country has any intention of cooperating with these new immigrants.
Increases in crime rates, raised as a concern by the critics of immigration, tend to be prevalent in places where immigration is subsidized by governments (i.e., case (2) above). This is often the case with so called political or asylum immigrants. These immigrants gain access to other countries by applying to those countries' government agencies. Once they get approved as political asylum seekers, they often end up as recipients of government welfare payments because the purpose of their arrival wasn't economic in the first place.
In this case, immigrants come to places where they may not welcomed by the home population. This may create tensions, even hatred, and conflict. The increases in crime rates are a consequence of the discoordination between the needs of the domestic population and government policies. The government simply lacks the time- and-place specific information on the needs of particular individuals within the home population, and how these needs match the desires and skills of the new immigrants.
Immigration that has its origins in private arrangements between the home population and immigrants (i.e., case (1) above) has far less potential to create conflict. An illustration of this is the reduction in crime rates in areas where Eastern European immigrants settled in Western Europe after the Eastern European countries joined the EU. These immigrants came to Western Europe because they saw new, unused employment opportunities, which were often communicated by family members, friends, or other potential business partners. These private arrangements were made only after all the involved parties have made sure immigration is a good fit for them. All the involved parties have the time- and place-specific personal information needed to find a good match between a new immigrant and the needs of the local community.
The argument about fewer jobs for the home population is another misconception. Economic logic implies that immigration creates new economic opportunities. People want to move to other places because they expect to find jobs there, except when they expect to be recipients of government welfare programs. People who make private employment arrangements tend to be entreprising individuals who avoid government welfare programs. These individuals generally create new jobs that have never existed before. These new jobs come into being due to the unique sets of skills, preferences, and entrepreneurial abilities that those particular immigrants posses.
For example, one of my first jobs as an immigrant was construction site clean-up. This was a new business model where a clean-up contractor sends crews of two people to construction sites. All clean-up crews in the Kitchener-Waterloo area were composed of new immigrants, mostly Serbian, Croatian, Bosnian, and Romanian. It is easy for me to understand why this business model would not be attractive to Canadians. The amount of income one could earn in this job was similar to the amount one could receive in welfare payments. None of us, however, wanted to live on welfare. We wanted to start vibrant, productive lives, and one does not start such a life on a welfare program. We wanted to show that we have the will to advance, the energy, persistence, and resilience needed to do any job. You don't show that by being on a welfare program.
It is true that these new unique jobs sometimes make some old "domestic" jobs obsolete. Our clean-up crews, for example, replaced some of the previous on-site personnel. But the only reason why these old jobs become obsolete is because the new jobs satisfy the needs of the home consumers and employers better. Thus, arguing that new immigrants should be barred from entering the country to work on their new jobs implies barring the domestic consumers and employers from enjoying the benefits of the unique skills that these new immigrants can provide.
There are certainly people who will, in the short run, be hurt by immigration. Those are people whose skills will become irrelevant after immigrants show they are more capable in certain activities. This implies that a portion of the home population will have to innovate and find new opportunities. Some people don't like to innovate, so they would rather vote for anti immigration laws.
Another worry of the critics is that new immigrants bring more burden on schools, hospitals, and public housing. This view is mistaken too. These services are provided by the government, and thus, people have a tendency to measure only the expense, and not the benefits. Were schools and hospitals private businesses, they would welcome new customers. Would you say that there is more burden on car dealerships because new immigrants want to buy more cars, or that there is more burden on the car repair shops because new immigrants want to fix their cars? Surely, if the owners of car dealerships and repair shops are willing to serve new immigrants, the benefits of serving them must outweigh the additional "burden".
Thus, we have to conclude that immigration that originates from private arrangements between the home population and new immigrants is beneficial to everyone, except to those who do not like to innovate, learn new skills, and find new opportunities. On the other hand, immigration organized by governments, which do not have the time- and place-specific information about the needs of the domestic population and the new immigrants, creates social discoordination and conflict. Immigration is a good thing if it is a consequence of voluntary exchange between consenting individuals.