Illustration of histamine. Histamine acts both as a hormone and as a neurotransmitter. As a neurotransmitter in the central nervous system, histamine influences the release of other neurotransmitters, the triggering of vomiting, and the regulation of the sleep-wake rhythm. Histamine plays an important role in the body's defense against foreign substances (immune system) but also in allergies and asthma. In addition, histamine is essential for inflammatory reactions (swelling of the tissue) and in burns.