I was born with a rare mass cell defect called urticaria pigmentosa. If someone so much as simply rubbed my skin my mass cells would go in overdrive and attack the area I was being touched, forming a permanent brown spot. By the time I was 8 my entire body was covered in brown spots. I was fortunate and outgrew it when I hit puberty but a lot of people end up with leukemia caused by urticaria pigmentosa. Some people do not form spots but instead form welts.