Classical class I and class II HLA proteins are both highly polymorphic heterodimers polymorphic heterodimers that help lymphocytes to recognize peptide antigens but they differ in many ways. Which, if any of the following statements, is true?

• A. The two chains of any class I HLA protein are made by genes that are located on different chromosomes.
• B. Each of the two protein chains of a classical class I HLA protein are highly polymorphic, unlike for classical class II HLA proteins where only one of the protein chains is polymorphic
• C. Class I HLA proteins assist helper T lymphocytes to recognize peptide antigens, whereas class II HLA proteins assist cytotoxic T lymphocytes to recognize peptide antigens.
• D. Class II HLA proteins are expressed on the surfaces of almost all nucleated cells, but the expression of class I HLA proteins is confined to just a few types of cell, notably certain immune system cells.