Immigration is a notoriously complex area of law, both because it is detailed and technical, and because it undergoes substantial changes almost every time the balance of power in Congress changes.
Immigration is an intensely political area of the law.
Under it, only Congress can pass laws regulating immigration, and it can pass laws that could not be applied to US citizens. The immigration laws passed by Congress are found in Title 8 of the United States Code. Since 1933, the INS has been part of the Justice Department. The INS is one of the largest federal agencies with over 29,000 employees. The rest are based in the service divisions of the agency.