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Orexin is a hypothalamic peptide neurotransmitter first described in 1998. Orexinergic neurons are located predominantly in hypothalamic structures and play an important role in regulating a variety of physiological functions. In the present article we review data on the locations of orexin-containing neurons in the brains of Wistar rats and their projections within the hypothalamus and to other CNS structures. The involvement of orexinergic neurons in regulating feeding behavior and sleep/waking cycles is reviewed, as is its involvement in controlling immune system functions. Experimental data providing a comparative analysis of the responses of orexin-containing neurons to antigenic and non-antigenic stimuli are presented, these providing evidence of the functional heterogeneity of hypothalamic orexin-containing neurons, which is particularly apparent in mediating the responses of the brain to antigenic and non-antigenic stimuli. Analysis of both changes in the intensity of the expression of the preproorexin gene and the morphofunctional characteristics of hypothalamic orexin-containing neurons after exposure to antigens provides grounds for their possible involvement in the mechanisms mediating the responses of the brains to antigens.