Based on their analyses, the researchers suggest autoimmune diseases fall into at least two different groups: one containing rheumatoid arthritis and ankylosing spondylitis and another containing multiple sclerosis and autoimmune thyroid disease.
I am not going to pretend to understand all the science on this one, but it's pretty interesting. Stanford University researcher Atul Butte worked with a large team to study meta data gathered "genome-wide" association data various autoimmune diseases to try to find ones that seem to go together or not.
They found there seems to be at least two types of autoimmune diseases, and having one of the types might actually reduce the risk of having others of the other type.
I bet I can guess that Crohn's disease belongs with the rheumatoid arthrisis and ankylising sponylitis side of the house. Why do I guess this? Because I happen to know that people with Crohn's have a higher risk of also having those two diseases. In fact rheumatoid arthritis and Crohn's share some biologic treatments such as Remicade and Humira.
I will be very interested in hearing more about this study. My personal theory is that the genetic component makes us susceptible to the disease, but the disease itself might be triggered by some environmental factor such as MAP bacteria.