A meta-analysis indicated that clinical depression is associated with several large alterations in cellular immunity. The authors, analyzing only methodologically sound studies, found that reliable (consistent) immune alterations included lowered proliferative response of lyphocytes to mitogens, lowered natural killer cell activity, and alterations in the numbers of several white blood cell populations. Immune alterations were greater in both older and hospitalized patients. There was also evidence of a linear relationship between intensity of depressive affect and indicators of cellular immunity. The authors discuss how neuroendocrine mechanisms or health practices might link depression to immunity. They also make suggestions about study design features that would elucidate better how these pathways are involved in mediating the relationship between depression and altered immunity.