Major Histocompatibility Complex (MHC) in Fish

The term “major histocompatibility complex” (MHC) was used originally to name a polymorphic genetic region in mammals, several loci of which were involved in determining acute allograft rejection, hence the term histocompatibility. “MHC molecules” commonly refers to a special class of proteins expressed at the surface of somatic cells and involved in several major immunological functions. This chapter starts with a brief history of the fish MHC research, then focuses on the particularities of the fish MHC genetic region in the context of multiple genome duplications, with the discovery of a great variability of its architecture and composition among the species. The structural and functional properties of MHC class I and class II molecules are then described. Their nomenclature and classification are explained in an evolutionary perspective. Classical and non-classical MHC molecules are examined across fish species in which they have been characterized. The role of MHC variation in resistance to diseases is also reviewed. Finally, the importance of MHC in mate choice is discussed.