The “self” you exist within is defined by what scientists now call “compatibility cells,” according to Mark Viney, a biologist at University of Bristol, UK. These cells are specific to the individual, and any cells incompatible with these cells are destroyed by the immune system. According to Viney, compatibility cells determine how you respond to infection, which is different for each person. The cells also control our choice for a mate. Immunologist Daniel Davis, author of The Compatibility Gene, covers the subject by looking over a series of studies that reveal human attractiveness is driven by compatibility genes.