During the past decade, a great deal of data has accumulated supporting the notion that cytokines interact to regulate several aspects of social and emotional behaviour. There are reports of a positive correlation between cytokine levels and aggressive behaviour in healthy populations, and clinical reports describe an increase of aggressive traits in patients who receive cytokine immunotherapy. Interleukin-1beta released during an immune response acts as messenger that helps to modulate behaviour by influencing relevant neurotransmitter systems, and in some cases, by directly acting within the brain. In this site, IL-1beta exerts its actions by acting through 5-HT2 and IL-1 Type I receptors in hypothalamus or by potentially indirect routes, including activation of sensory afferents, and stimulation of cytokine release by brain endothelial cells. This review reports research investigating the relationship between IL-1beta, and the immune and central nervous systems involving or potentially involving defensive aggressive behaviour.