According to Kennedy why is it in possible to speak about particular immigrant contribution to the United States

Kennedy wrote his book A Nation of Immigrants in 1963, seeking to promote pro-immigrant reforms, and addressed all the arguments that opponents were throwing against immigrants. Kennedy says that all Americans are immigrants and talk about preferring one another, it's racism. He speaks of the Mexicans, and said that they are attacked in the USA, just like the Irish in the 1920s. Therefore, his main idea is that Americans should remember their history as immigrants, to build a strong and united nation.