The brain does not operate in a vacuum : in close contact with the body, it is in constant dialogue with the immune system. These interactions, long considered anecdotal or linked solely to pathological situations, are now recognized as fundamental to development, homeostasis and responses to aggression. The nervous and immune systems share molecular and cellular communication pathways, enabling fine coordination of physiological responses. This lecture, the first part of a two-year cycle, introduces the fundamentals of these neuroimmune interactions, with a particular focus on molecular mechanisms, peripheral perception systems, and the ways in which the brain interprets immune signals as a new sense : immunoception.