As much as the two diseases mimic one another, in symptoms and lab results, they are different disease entities, both autoimmune. There is confusion about which comes first, because of the similarities, but there is no "standard" as to which one follows the other.
Part of the confusion is that if you have SjS and NO OTHER autoimmune disease, the SjS is referred to as primary, but if you have another AI, like Lupus, the SjS is called secondary. This does not mean that it came second, or that it is secondary in importance or severity, it just means that it is a another AI in a group.
Another way of looking at the primary /secondary phrasing is that once you have more than one AI, they are all secondary to a primary autoimmune dysfunction.
Often the SjS (being deemed by doctors to be a less severe and more localized process) is diagnosed after a diagnosis of Lupus, because the Lupus is much more obvious. In other cases, Someone with SjS has more "extra-glandular symptoms" (not just dry eye/mouth) and the doctors continue to look until they can diagnosis Lupus, which they feel justifies the more severe symptoms.
