On the basis of this and other findings such as increased numbers of CD3+ and CD4+ cells, an increased ratio of CD4+/CD8+ cells, and a reduced level of suppressor cell activity in schizophrenia and endogenous depression, we investigated the influence of the human leukocyte antigen-Class I (HLA-A, HLA-B, HLA-C) system on the altered immune function and evaluated the relationship to immune function of a family history of psychiatric disorders.