Allergists focus on traditional medical issues of acuity, life and death. IgG issues most often arise in the evaluation of chronic illness that often presents as refractory mental problems - like ADHD.
Just as when traveling in any new territory we need useful new maps to find the towns we will be visiting. Many of these towns show up on the map with names we haven't heard before - and those names can be down right intimidating - even to a psychiatrist:
If you think you have it bad with ADHD, just throw picky eating into the problem. Then ADHD can become almost untreatable. Those with picky eating problems often [not always!] have significant problems with the North of the GI tract and immunity which gums up the works.