People with bipolar tend to have higher antibodies to gliadin (gluten) than controls. People with schizophrenia have higher casein (dairy) antibodies than controls. In neither case does it cause the illness, but in both in can impact severity of symptoms.
http://www.ncbi.nlm.nih.gov/pubmed/21320252 And
http://www.ncbi.nlm.nih.gov/pubmed/22801085. It does not mean it applies to everyone, but scandinavian/Northern European people with milder cases of both illness often actually have autoimmune disease. One study found 7percent of schizophrenia inpatients actually had Celiac, to say nothing of other autoimmune disorders. More:
http://www.celiac.com/categories/Celiac--Disease--Research.....