The proteins on the outer part of body cells that are (effectively) unique to that person. Any cell displaying that person's HLA type belongs to that person (i.e., is not an invader). Any cell displaying some other HLA type is "not-self" and is an invader. HLA types are inherited, and some of them are connected with autoimmune disorders and other diseases. Scientists think that people with certain types of HLA antigens are more likely to develop it.
HLA types are the key to matching tissues and organs for donation and transplant.