Using mice, researchers have demonstrated a link between an individual’s immune system and the susceptibility to depression. Employing bone-marrow transplants to study the effects of Interleukin-6 (IL-6) levels in the rodents, they found evidence of a strong correlation between higher levels of the cytokine and a propensity toward depression. This suggests a possible mechanism involved in the observed tendency for susceptibility to depression being hereditary. Ars Technica has the details: