Research a year earlier on depression and bipolar disorder also found higher levels of IL-1β, TNF-α, soluble tumour necrosis factor receptor (sTNFR)1, IL-12 and IL-10 in patients with MDD, compared with higher levels of IL-6, sTNFR2, IL-18, IL-33, soluble suppression of tumourigenicity 2 and neuroprotective gene modulator KLOTHO in patients with bipolar disorder.61 Here, IL33 is linked to major depressive disorder.