There is much more evidence suggesting very close cooperation between nervous and immune systems. This cooperation is regulated by soluble factors released by the cells of both systems: cytokines and neuropeptides. Classical neuropeptides can be released by cells of the immune system, whereas a variety of cytokines produced by cells of the immune system are synthesized and released by nervous tissue and endocrine glands too. Specific receptors for these moleculary different families of soluble factors are present on both the immune and nervous systems cells.