IFNG and major depressive disorder: The primary analysis will estimate the relationship between psychosocial variables (e.g. depression, stress, affectivity, meaning in life, well-being); between psychosocial indices and inflammatory biomarkers (e.g. anxiety and IL-6; stress and IFNγ; coping skills and pro or anti-inflammatory cytokines; positive affect and TNF-α; spiritual well-being and IL-2); and, between psychosocial factors and cytological parameters (e.g. stress/depression/unhealthy behaviors and the number of NK and cytotoxic T cells; positive affect/adjusted coping skills and cortisol).