The sources of received noise are usually outside the system, and therefore out of reach for the system designer. The environment is what it is, and the first possibility for the system designer to do something about the noise is on the system inputs and on the power cables. For a system delivered with dedicated cables, it is even possible to start on the cable itself. A good example here is a computer monitor, where you quite often see a filter put next to the VGA plug you connect to your computer. On other systems, the first chance comes with the I/O connectors. For a hand-held, battery-powered application without any cables, this is not applicable, but then this problem is similarly smaller. If external noise can be prevented from entering the system at all, there will be no immunity problems.