Twin boys with major histocompatibility complex class II deficiency but inducible immune responses

The major histocompatibility complex (MHC) class II molecules are composed of two polypeptide chains (α and β). In humans the three distinct isotypes are designated HLA-DR, DP, and DQ. The different isotypes serve similar functions, and their expression on cells is regulated in coordinated fashion.1 MHC class II molecules are expressed on the surface of certain cells of the immune system, such as monocytes and macrophages, B cells, and dendritic cells, where they play an important part in the immune response.2 MHC class II deficiency is an autosomal recessive disease characterized by cellular and humoral immunodeficiency.3 Family studies show that . . .