There are many reasons to spay and neuter your pets. Spay/neuter is the best way to reduce homeless pet overpopulation and to prevent certain kinds of diseases—including cancer—of the reproductive system in both males and females.
New research published in the Journal of Allergy and Clinical Immunology suggests that another benefit of neutering is to reduce human allergic reactions to a specific protein found in intact male dogs.
Dog and cat allergies are common, and can be caused by allergens in your pet’s urine, saliva, and dander. But this new research shows that as many as 30% of people with dog allergies may be affected by a single prostate protein called Can f 5, found only in male dogs.
People who are allergic to Can f 5 can live with female dogs in the house but have problems with male dogs. Some people with this allergy can tolerate neutered male dogs, while others may be better off with female dogs only.
If you suffer from pet allergies, you can note if certain dogs are easier to be around than others. You can also talk to your doctor about getting tested. An article on the Can f 5 research on the Mother Nature Network website notes that there are currently 6 different dog protein tests available, so you can narrow down the exact cause of your pet allergy.
FACE Foundation Blog 
That answers a lot of questions.
Glad to pass this along.
Wow, I had no idea. That was really fascinating.
Yes, interesting!