The neuroendocrine and immune systems are bidrectionally linked. Products of the immune system like lymphokines and monokines modulate the neuroendocrine functions while various neuroendocrine signals also affect the immuune functions. Neuroendocrine and lymphoid cells share a number of neurotransmitters, neuromodulatory substances, hormones and their receptors suggesting the existence of a bidrectional regulation between the two systems. Evidence shows that such a reciprocal interaction exists between the pineal gland and the immune system.