Doctors have discovered that up to 30 percent of dog-allergic patients are only sensitized to one specific protein made in the prostate of male dogs. In other words, pet owners with allergies might be able to tolerate female dogs or male dogs that have been neutered.
Source: Animal Wellness, February 3, 2020. Link.
Also see: Allergic to dogs? It may only be the males, CNN, January 15, 2020. Link.