Itches are triggered by a special neurotransmitter called Nppb, which also regulates and controls our blood pressures.
Gil Yosipovitch, who is the director of the Temple Itch Center (!), says that itches evolved to warn us of dangers in our environment.
The receptors on our skin surface react to the tiniest disturbance, including dust, soap residue, dead skin, or a thread of clothing. Whatever it is, it’s the neurotransmitter’s job to tell us there’s a disturbance!Src: mental_floss