Evidence of altered immune function and inflammatory signaling has been reported in samples of individuals affected by major psychiatric and neurodevelopmental disorders; it remains unclear how these altered immunological states arise, though a genetic basis has been postulated. The present study sought to use existing summary-level data generated from previous genome-wide association studies (GWAS) in order to explore whether common variant genetic risk factors might be shared between a set of psychiatric/neurodevelopmental phenotypes and a set of medical phenotypes enriched with immune and inflammatory processes. Based on the available GWAS summary data, we calculated the estimated heritability for each phenotype and we examined the genetic correlations between pair-wise combinations of phenotypes, using the LD Score Regression method. We observed positive genetic correlations between bipolar disorder and both celiac disease (rg = 0.31 + 0.09) and ulcerative colitis (rg = 0.25 + 0.06), which survived correction for multiple testing. We also observed several robust genetic correlations amongst the set of medical phenotypes enriched for immune and inflammatory processes. We review the relevant clinical literature and suggest that similarities in common variant genetic diatheses may contribute to increased comorbidity between bipolar and autoimmune/inflammatory conditions involving the gastrointestinal tract. We also discuss the limitations of the present approach and important caveats for interpreting the findings.