There are also “tumor sniffing dogs” who can find cancer when other more “conventional” methods fail.
Their incredible sense of smell is probably the reason why they are so successful.
Minute changes in the chemistry of their owners, unnoticed by humans would stand out like red flags of alarm, to them.
That, of course, is true, but knowing the medical details as to the why of the danger to the dog owner is a bit beyond the pale. I doubt if a dog can understand the concept of an allergy or hemophilia. They sense danger and they respond.