Recent research has demonstrated a relationship between depression and immune system activity, specifically proinflammatory cytokine activity. Although experimentally-induced immune activation leads to increases in depressed mood, the neural correlates associated with these changes have remained largely unexplored. Based on relationships between cytokine activity, depression, and heightened physical and social pain sensitivity, I propose to investigate the effect of proinflammatory cytokine activation on the neural correlates of socially painful experience that may contribute to depression. Our previous work has shown that the dorsal anterior cingulate cortex (dACC), typically associated with physical pain distress, also plays a role in the distressing feelings associated with social rejection or social loss. Moreover, recent pilot data has revealed that individuals with elevated levels of baseline proinflammatory cytokines report feeling more distressed and show more dACC activity during social rejection. To investigate the causal role that cytokines may play in the heightened social pain sensitivity that can contribute to depression, participants will be randomly assigned to receive either endotoxin (which increases proinflammatory cytokine activity) or placebo. Subsequently, participants will complete a neuroimaging study in which they will be rejected during an online ball-tossing game. We hypothesize that individuals exposed to endotoxin will report more social distress and depression following rejection and will show more dACC reactivity during rejection. The proposed study is the first to investigate the effect of systemic inflammation on neural reactivity related to social and affective processes that may increase the risk of depression.