These results provide the first evidence that changes in optimistic expectancies are accompanied by changes in immunity, as well as the first evidence for a mechanism by which this effect occurs. Changes in expectancies about law school predicted changes in cellular immune function, and this relationship could be partially accounted for by positive but not negative affect. The results support the validity of psychological interventions to improve immunity and health (e.g., Andersen et al., 2007) and suggest that efforts to correct irrationally pessimistic expectancies may be warranted, particularly if these efforts also increase positive affect. Previous investigations into the effects of expectancies on immunity and health have not found strong evidence for affect as a mediator of these effects. However, these investigations examined only differences between people and not changes within people, and they also failed to separate the effects of positive and negative affect, which may be largely independent of each other (Feldman Barrett & Russell, 1999; Watson & Tellegen, 1985). The present results support assertions and empirical evidence that the health effects of positive affect, such as greater longevity and better neuroendocrine and immune function, are more than the absence of negative affect (Danner, Snowdon, & Friesen, 2001; Lyubomirsky, King, & Diener, 2005; Pressman & Cohen, 2005; Stone, Cox, Valdimarsdottir, Jandorf, & Neale, 1987). In this case, negative affect could not account for any of the relationship between optimistic expectancies and immunity.