Most of the genes in the major histocompatibility complex (MHC) region express high polymorphism that is fundamental for their function. The most important function of human leukocyte antigen (HLA) molecule is induction, regulation of immune responses and the selection of the T cell repertoir. A clinician’s attention is normally drawn to a system only when it malfunction. The HLA system is no exception in this regard, but in contrast to other systems, it is also arouses interest when it functions well – too well, in fact. population studies carried out over the last several decades have identified a long list of human diseases that are significantly more common among individuals that carry particular HLA alleles including inflammatory, autoimmune and malignant disorders. HLA disease association is the name of this phenomenon, and the mechanism underlying is still a subject of hot debate. Social behaviours are affected by HLA genes and preference for HLA disparate mates may provide “good genes” for an individual’s offspring. Also certain HLA genes may be associated with shorter life and others with longer lifespan, but the effects depend both on genetic background an on the environtmental condition.