Which of the following BEST represents the relationship of polypeptide units in MHC class II?
A. a and ß chains from any gene can pair with each other.
B. Only a and ß chains from the same gene can pair with each other.
C. Only a and ß chains of the same allele can pair with each other.
D. a chains can pair with other a chains, but ß chains can only pair with a chains.
E. a and ß chains pair until the complex is exported to the cell surface and can bind ß-2 microglobulin.
Answer: B