Within the endocrine system, specificity of communication is determined by
Hormones produce their effects on target cells by interacting with specific receptors. Hormone binding to its receptor generally initiates a cascade of events that lead to biological effects in the target cells.
The chemical nature of the hormone
The distance between the endocrine cell and its target cell(s)
The presence of specific receptors on target cells
Anatomical connections between the endocrine and target cells
General physiology