The major histocompatibility complex (MHC) in chickens was initially described as a blood group locus called the B system. Early studies on the B blood group system re-vealed that the B system was associated with some physiological functions. Subsequently, it was found that the B system was associated with several immunological functions and should be the MHC in chickens.Then, the system is now called the B complex. The B complex is composed of three regions, B-G, B-F and B-L, and regulates a number of functions similar to those found in mammalian MHC. The DNA structure of each region within the B complex is now gradually becoming clear.