IL6 and depressive symptom measurement: For example, elevated plasma levels of interleukin-6 (IL-6)—a ‘keystone’ cytokine involved with both pro- and anti-inflammatory signalling (Hunter and Jones, 2015)—are consistently reported in people with depression (Khandaker et al., 2018; Osimo et al., 2020), but are also present in eating disorders (Dalton et al., 2018), psychosis, and post-traumatic stress disorder (PTSD) (Lindqvist et al., 2014; Passos et al., 2015).