From the article:
A study has found that cancer patients suffering depression have decreased amounts of brain-derived neurotophic factor (BDNF) in their blood. Low levels make people less responsive to cancer drugs and less tolerant of their side-effects.
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Researchers found that BDNF clearly boosted the number of tumour cells killed by chemotherapy. Patients with severe depression had lower levels of the protein in the blood so their bodies were not as effective at fighting cancer. This reduced their chance of surviving the disease