Hello, Thank you for asking your query at DoctorSpring.

Allergy is a reaction your body develops to substances in the environment your body feels harmful. The very common reason people develop allergy with cats is to the cat fur or dander. Dander is nothing but the dead skin cells that has been shed. You had specified that you have more than one cat in your house. Therefore than in your friends house who has only one cat, there is increased collection of dander and fur in your house. Cleaning the dander and fur iin your friends house is easier obviously because there is just one cat. But, in your house on the other hand, dander and cat fur is built up in your house and covers the chairs, rugs, etc because of many cats.

When you walk around or switch on the fan or air conditioner, the collected dander is stirred up and enters your nose. Whereas, in your friends house you had specified there is only one cat, the management is easier and the amount of fur and dander collected will also be less comparatively to produce an allergic reaction. That is the reason you do not develop allergies in your friends house.

Hope this helps.

Please feel free to ask follow-up questions