Cell-cell communications involve signaling molecules from one cell interacting with receptor proteins in another. When both signaling and receptor molecules are present on cell surfaces, they can interact only when the two cells come into close contact -- this type of signaling is known as juxtacrine signaling.
If the signal is released from one cell and travels only a limited distance, it can interact with receptors located in cells within a local neighborhood, the signal is said to be paracrine.
When cells release a signal into the bloodstream, that signal can move throughout the body. Cells that have the appropriate receptors, no matter where they are, can react to the signal -- such signaling is known as endocrine.
Exocrine glands are composed of cells that secrete materials into the exterior world -- they are involved in interactions between organisms. Pheromones are a good example.
