Cell-to-cell communication is classified by the distance a signal travels and the size of the audience it reaches, yielding four modes: direct contact (juxtacrine) signaling, in which membrane-bound proteins of two touching cells physically engage; neural signaling, in which a chemical transmitter diffuses across a narrow synaptic gap to a single designated target; paracrine signaling, in which a released chemical messenger acts locally on a neighboring group of cells; and endocrine signaling, in which a hormone is released into the bloodstream and is potentially available to every cell of the body. The unifying principle is that specificity of response is determined by receptor complementarity rather than by the reach of the signal — a broadcast message affects only those cells bearing the appropriate binding protein. This concept belongs to cell biology and physiology, provides the organizing framework for immunology, neuroscience, and endocrinology, and is invoked to explain how intercellular communication makes multicellular complexity possible.
Cell-to-cell communication is classified by the distance a signal travels and the size of the audience it reaches, yielding four modes: direct contact (juxtacrine) signaling, in which membrane-bound …