In the 1940's, Mitsuda identified a clinical patient group with the features of acute onset, hallucination, and/or mood disturbance in the acute phase. Patients with this type of disorder worsen periodically, but they can lead a normal life after such periods of short-duration exacerbation, and may often exhibit memory disturbance regarding the duration (Mitsuda, 1942). Compared with typical schizophrenia, the course and outcome are different in these atypical cases. In addition, epidemiological genetic studies found that typical schizophrenia and atypical cases showed different clinical outcomes even in the same families;therefore, he proposed the clinical concept of "atypical psychosis" or "Mitsuda psychosis", consisting of these atypical features.
 Using modern medical knowledge, similarities of clinical features between atypical psychosis and autoimmune diseases, such as CNS lupus derived from SLE or anti-NMDA receptor encephalitis, can be identified. The atypical psychosis proposed by Mitsuda can be partly explained by an autoimmune mechanism. If it is a clinical group based on encephalitis, epileptic discharge or abnormalities in EEG can easily be observed, and various psychotic symptoms or consciousness disturbance can also be seen if an inflammatory response is widespread in the brain.
 While autoimmune disease is related to the MHC genetic region on Chromosome 6, our group identified high susceptibility related to the MHC region on GWAS analysis for atypical psychosis. It has been reported that SLE is frequently seen in females and non-Caucasians because the susceptible HLA type for SLE is frequent in them. Mitsuda's atypical psychosis is diagnosed as "brief psychotic disorder(298.8)" or "schizophreniform disorder (295.40)" according to DSM-5, and these disorders are known to be seen more frequently in females or in developing countries. If the reason is partly based on the low prevalence of Caucasians in these countries, it may be possible to consider that a certain number of people suffering from these acute mental disorders have a similar etiology involving autoimmune encephalitis.
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