Nitric oxide (NO) has an important role in physiological and pathological processes in general, and in particular plays a homeostatic
role in the nervous and immune systems. The many different physiological functions of NO include those of a mediator of
blood vessel dilation, neurotransmitter, neuromodulator and inductor of mitochondrial biogenesis. In addition, NO can transform into
highly reactive and harmful molecules producing an impairment of the DNA, lipids or proteins, and thus altering their function. This dual
action of NO, by which it plays an important role in homeostasis and aids the development of pathological processes, makes this molecule
an interesting target for medical therapies, especially with respect to the nervous and immune systems. This review describes the
multiple roles of NO played out in the nervous and immune systems during different physiological and pathophysiological processes.