I ran across this website when I was trying to find the name of a doctor in another state....  I think what they've done is to correlate all diseases with specialties who treat them and to list all doctors in those specialties.  In the case of mast cell disease, they've listed hematologists only

This is the type of site to be wary of finding a real mast cell expert.  I looked through Colorado's list and have never heard of any of them.  The doctors who do know about mast cell disorders aren't listed.  Dr. Akin isn't listed in Boston!

http://www.lifescript.com/doctor-directory/condition/m-mast-cell-diseases