What makes us allergic to things?

Answer 1

Sometimes it comes from a family of people with the same thing and how their ancestors had to adapt to their original land or sometimes its from a fear of trying it that makes your body act different. I hope that helped if that didn't then I have no other way to explain it.

Answer 2

Usually, especially with cats and dogs, it's saliva or the dander (dead skin that is constantly shed) that people react to. Saliva is spread by cats/dogs cleaning themselves then shedding fur. Allergies can affect anyone at any age. Some worse than others.
The Human Respiratory system in is not protected from such dust and hair that might fall off of pets. Esp in case of people who do not have enough nasal hair. This might cause respiratory problems like suffocation, sneezing etc to those who are allergic to the dust.

Answer 3

All allergies are caused by the over reaction of the immune system to a stimulant. In the case of cats this can be such things as

Hair

Skin flakes

Mites or paracite feces or bodies

cat feces or urine