What led to the fomentation of labor unions during the second industrial revolution?

Unprecedented immigration of people from Europe during the late 1800s increased the pool of people asking for jobs despite of terrible labor conditions. Since Civil Wars, no federal intervention over people rights, safety regulations, working hours or minimum wages were improved over the States. Meanwhile, throughout Europe, in lot of countries were appearing various forms of socialism, and that ideas came with inmigrants, even though they weren't unsatisfied with their jobs, but they just wanted better conditions of laboring, less arbitrarily and most security.