What we're describing is not an immune deficiency, but rather an immune dysfunction. Many (although not all) seem to share a history of ear infections and spitting up as babies (possibly milk-related,) or of chronic diarrhea, constipation, or loose stools (possibly wheat-related.)
Other parents note that their autistic children seem to be the healthiest members of the family. In this case, it has been hypothesized that the immune system is too aggressive and ends up turning on the nervous system. This may explain the presence of anti-myelin antibodies in some children, and may also explain why some have immune issues like multiple allergies but do not respond well to dietary intervention.