"A personal history and a maternal history of autoimmune disease were associated with an increased risk of ADHD [attention-deficit hyperactivity disorder]. The previously reported association between type 1 diabetes and ADHD was confirmed. In addition, specific parental autoimmune diseases were associated with ADHD in offspring."
So said the study results published by Nielsen and colleagues  including "a study population of 983,680 individuals followed from 1995 to 2012." Reliant once again on one/some of those marvellous Scandinavian population registries (every country should have them), researchers "investigated the association between a personal history and a family history of autoimmune disease and the risk of developing attention-deficit/hyperactivity disorder (ADHD)." Autoimmune disease refers to the development of symptoms when the body's own immune system fails to recognise 'self' as 'self' and starts attacking its own tissue(s). I might add that the authors on the Neilsen paper have some research history when it comes to the behavioural correlates of autoimmune disease (see here).
Drawing on data for around 23,000 children diagnosed with ADHD, researchers reported a modest but significant 'risk' of ADHD being diagnosed where an autoimmune condition was reported. The presence of an autoimmune condition being diagnosed in mums also elevated the risk of offspring being diagnosed with ADHD (again modestly but significantly). Further: "a paternal history of autoimmune diseases was not significantly associated with ADHD in the offspring." Various types of autoimmune disease in the immediate family were potentially associated with offspring ADHD diagnosis: "a family history of thyrotoxicosis, type 1 diabetes, autoimmune hepatitis, psoriasis, and ankylosing spondylitis." This follows other population registry studies in this area .
Based on the Nielsen and other data, I don't yet think we're in the realm of calling something as diverse as ADHD an autoimmune condition on the basis that autoimmune conditions tend to 'flock together' . But I do think this adds to a growing body of literature talking about immune function and ADHD (see here for example) and some quite 'strong' data (see here). Hopefully without being too speculative, I'd also draw your attention to some interesting work talking about some possible reasons why autoimmunity might be over-represented when it comes to [some] ADHD on the basis of the expression of those fossil viruses that we all have and their 'super-antigen' and 'molecular mimicry' properties. Just one possibility among many...
Music: Nana Mouskouri sings Hey Jude...
 Nielsen PR. et al. Associations Between Autoimmune Diseases and Attention-Deficit/Hyperactivity Disorder: A Nationwide Study. J Am Acad Child Adolesc Psychiatry. 2017 Mar;56(3):234-240.e1.
 Instanes JT. et al. Attention-Deficit/Hyperactivity Disorder in Offspring of Mothers With Inflammatory and Immune System Diseases. Biol Psychiatry. 2017 Mar 1;81(5):452-459.
 Assa A. et al. Large population study shows that adolescents with celiac disease have an increased risk of multiple autoimmune and non-autoimmune comorbidities. Acta Paediatr. 2017 Mar 1.
Nielsen PR, Benros ME, & Dalsgaard S (2017). Associations Between Autoimmune Diseases and Attention-Deficit/Hyperactivity Disorder: A Nationwide Study. Journal of the American Academy of Child and Adolescent Psychiatry, 56 (3), 234-2400 PMID: 28219489