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Studies at UCLA found that optimistic beliefs affect key immune cells and mood. Those who felt confident about their abilities and expectations of success had more helper T cells, which support immune responses; and more effective natural killer cells, which destroy substances that are poisonous to cells. (Segerstrom, S.C., et al. “Optimism is associated with mood, coping, and immune change in response to stress.” Journal of Personality and social Psychology. 74(6):1646-1655, 1998)

Optimistic people have been found to secrete less cortisol, a stress-related hormone that suppresses immune system function. Lower cortisol levels make the immune system function more effective. (Lai, J.C., et al. “Optimism, positive affectivity, and salivary cortisol.” Journal of Personality and social Psychology. 10 (Part 4):467-484, 2005)