“The majority of genes in the [MHC (Mean Histocompatibility Complex)] class I region are not involved in the immune system; neither do the class I genes form as a compact cluster as the class II genes. Whereas genes encoding class II molecules are only present in the class II region of the HLA [(Human Leukocyte Antigen) complex], genes encoding class I molecules and related class I-like molecules are found on several different chromosomes. A further difference is that HLA class II molecules are dedicated components of adaptive immunity that serve only to present antigen to T cells, whereas HLA class I and class I-like molecules encompass a broader range of functions, including uptake of IgG in the gut, regulation of iron metabolism, and regulation of NK [(Natural Killer)]-cell function in the innate immune response. Together, these genetic and functional differences show that MCH class I is the older form of MHC molecule and that MHC class II evolved more recently from MHC class I. Consistent with this proposition, MHC class II is not always essential for a vertebrate immune system, unlike class I. The Atlantic cod manages very well with only MHC class I genes.”