How can an adult develope allergys to mold, pollen, dust mites and grass?

Gene & environment: When your genes and the changing environment intreacts, some people would develop allergies. Allergy develops because your immune system has been redirected to react to otherwise innocuous substances such as animal dander, pollen, dust mite,mold, food etc. The stronger a family history of allergic diseases, the more likely you are going to become allergic.