Which of the following statements best describes a cytokine?
a) A type of lymphocyte that can induce apoptosis, or programmed cell death.
b) A specialized granulocyte that circulates in the blood.
c) An enzyme released during degranulation.
d) A signaling molecule that allows cells to communicate with each other over short distances.
e) A soluble chemical mediator whose function is to attract cells (chemotaxis) from longer distances.