Suicide as a Systemic Disorder

Suicide is a multifaceted phenomenon, related to an everlasting interconnection between biological, psychosocial, sociological, philosophical, and cultural aspects of human life. Immune system, which is constantly responding to changing environments and changing in response to them, seems to play a major role in this interaction. Multiple studies have reported increased risk of suicidal behaviours after or during inflammatory conditions. Individuals that engage in suicidal behaviours also have increased concentrations of inflammatory markers, such as interleukins 1β and 6, tumour necrosis factor α, and C-reactive protein, both in blood and the central nervous system (CNS). Stress, which is frequently mentioned among the major risk factors for suicidal behaviours, is also associated with a wide immune system dysregulation, entailing glucocorticoid system disruption and a low-grade inflammation. Mechanisms connecting systemic inflammation and CNS changes include damage to blood-brain barrier and interoceptive alterations, both resulting in changes in the communication between the periphery and the brain. Meanwhile, in CNS, pro-inflammatory cytokines may activate microglia causing a shift in tryptophan metabolism, preferentially generating cellular energy and toxic by-products of kynurenine pathway rather than serotonin. These changes in brain homeostasis lead to anatomical and functional brain alterations, most notably prefrontal cortex and insula, engendering maladaptive behavioural phenotypes, such as anhedonia and impulsivity that may mediate the association between inflammation and suicide. This chapter will provide a condensed overview of this complex association between inflammation and suicidal behaviours, presenting major findings in this area and explaining key pathways linking them.